Four years before the death of Thorlogh O'Brien, a Prince destined to be the life-long rival of his great son, had succeeded to the kingship of the northern tribes. This was Donald, son of Ardgall, Prince of Aileach, sometimes called "O" and sometimes "Mac" Laughlin. Donald had reached the mature age of forty when he succeeded in the course of nature to his father, Ardgall, and was admitted the first man of the North, not only in station but for personal graces and accomplishments; for wisdom, wealth, liberality, and love of military adventure.

Murkertach, or Murtogh O'Brien, was of nearly the same age as his rival, and his equal, if not superior in talents, both for peace and war. During the last years of his father's reign and illness, he had been the real ruler of the south, and had enforced the claims of Cashel on all the tribes of Leath Mogha, from Dublin to Galway. In the year 1094, by mutual compact, brought about through the intercession of the Archbishop of Armagh and the great body of the clergy, north and south - and still more perhaps by the pestilence and famine which raged at intervals during the last years of the eleventh century - this ancient division of the midland asker, running east and west, was solemnly restored by consent of both parties, and Leath Mogha and Leath Conn became for the moment independent territories. So thoroughly did the Church enter into the arrangement, that, at the Synod of Rath-Brazil, held a few years later, the seats of the twelve Bishops of the southern half were grouped round the Archbishop of Cashel, while the twelve of the northern half were ranged round the Archbishop of Armagh. The Bishops of Meath, the ancient mensal of the monarchy, seem to have occupied a middle station between the benches of the north and south.

Notwithstanding the solemn compact of 1094, Murtogh did not long cease to claim the title, nor to seek the hostages of all Ireland. As soon as the fearful visitations with which the century had closed were passed over, he resumed his warlike forays, and found Donald of Aileach nothing loath to try again the issue of arms. Each prince, however, seems to have been more anxious to coerce or interest the secondary chiefs in his own behalf than to meet his rival in the old-style pitched battle. Murtogh's annual march was usually along the Shannon, into Leitrim, thence north by Sligo, and across the Erne and Finn into Donegal and Derry. Donald's annual excursion led commonly along the Bann, into Dalriada and Ulidia, Whence by way of Newry, across the Boyne, into Meath, and from West-Meath into Munster. In one of these forays, at the very opening of the twelfth century, Donald surprised Kinkora in the absence of its lord, razed the fort and levelled the buildings to the earth. But the next season the southern king paid him back in kind, when he attacked and demolished Aileach, and caused each of his soldiers to carry off a stone of the ruin in his knapsack. "I never heard of the billeting of grit stones," exclaims a bard of those days, "though I have heard of the billeting of soldiers: but now we see the stones of Aileach billeted on the horses of the King of the West!"

Such circuits of the Irish kings, especially in days of opposition, were repeated with much regularity. They seem to have set out commonly in May - or soon after the festival of Easter - and when the tour of the island was made, they occupied about six weeks in duration. The precise number of men who took part in these visitations is nowhere stated, but in critical times no prince, claiming the perilous honour of Ard-Righ, would be likely to march with less than from five to ten thousand men. The movements of such a multitude must have been attended with many oppressions and inconveniences; their encampment for even a week in any territory must have been a serious burthen to the resident inhabitants, whether hostile or hospitable. Yet this was one inevitable consequence of the breaking up of the federal centre at Tara. In earlier days, the Ard-Righ, on his election, or in an emergency, made an armed procession through the island. Ordinarily, however, his suffragans visited him, and not he them; all Ireland went up to Tara to the Feis, or to the festivals of Baaltine and Samhain. Now that there was no Tara to go to, the monarch, or would-be monarch, found it indispensable to show himself often, and to exercise his authority in person, among every considerable tribe in the island. To do justice to Murtogh O'Brien, he does not appear to have sought occasions of employing force when on these expeditions, but rather to have acted the part of an armed negotiator. On his return from the demolition of Aileach (A.D. 1101), among other acts of munificence, he, in an assembly of the clergy of Leath Mogha, made a solemn gift of the city of Cashel, free of all rents and dues, to the Archbishop and the Clergy, for ever. His munificence to churches, and his patronage of holy men, were eminent traits in this Prince's character. And the clergy of that age were eminently worthy of the favours of such Princes. Their interposition frequently brought about a truce between the northern and southern kings. In the year 1103, the hostages of both were placed in custody with Donald, Archbishop of Armagh, to guarantee a twelvemonth's peace. But the next season the contest was renewed. Murtogh besieged Armagh for a week, which Donald of Aileach successfully defended, until the siege was abandoned. In a subsequent battle the northern force defeated one division of Murtogh's allies in Iveagh, under the Prince of Leinster, who fell on the field, with the lords of Idrone, Ossory, Desies, Kerry, and the Dublin Danes. Murtogh himself, with another division of his troops, was on an incursion into Antrim when he heard of this defeat. The northern visitors carried off among other spoils the royal tent and standard, a trophy which gave new bitterness on the one side, and new confidence on the other. Donald, the good Archbishop, the following year (A.D. 1105) proceeded to Dublin, where Murtogh was, or was soon expected, to renew the previous peace between North and South, but he fell suddenly ill soon after his arrival, and caused himself to be carried homewards in haste. At a church by the wayside, not far from Dublin, he was anointed and received the viaticum. He survived, however, to reach Armagh, where he expired on the 12th day of August. Kellach, latinized Celsus, his saintly successor, was promoted to the Primacy, and solemnly consecrated on Saint Adamnan's day following - the 23rd of September, 1105.

Archbishop Celsus, whose accession was equally well received in Munster as in Ulster, followed in the footsteps of his pious predecessor, in taking a decided part with neither Leath Mogha nor Leath Conn. When, in the year 1110, both parties marched to Slieve-Fuaid, with a view to a challenge of battle, Celsus interposed between them the Bachall-Isa - and a solemn truce followed; again, three years later, when they confronted each other in Iveagh, in Down, similar success attended a similar interposition. Three years later Murtogh O'Brien was seized with so severe an illness, that he became like to a living skeleton, and though he recovered sufficiently to resume the exercise of authority he never regained his full health. He died in a spiritual retreat, at Lismore, on the 4th of the Ides of March, A.D. 1119, and was buried at Killaloe. His great rival, Donald of Leath Conn, did not long survive him: he died at Derry, also in a religious house, on the 5th of the Ides of February, A.D. 1121.

While these two able men were thus for more than a quarter of a century struggling for the supremacy, a third power was gradually strengthening itself west of the Shannon, destined to profit by the contest, more than either of the principals. This was the family of O'Conor, of Roscommon, who derived their pedigree from the same stock as the O'Neils, and their name from Conor, an ancestor, who ruled over Connaught, towards the end of the ninth century. Two or three of their line before Conor had possessed the same rank and title, but it was by no means regarded as an adjunct of the house of Rathcrogan, before the time at which we have arrived. Their co-relatives, sometimes their rivals, but oftener their allies, were the O'Ruarcs of Breffny, McDermots of Moylurg, the O'Flahertys of Iar or West Connaught, the O'Shaughnessys, O'Heynes, and O'Dowdas. The great neighbouring family of O'Kelly had sprung from a different branch of the far-spreading Gaelic tree. At the opening of the twelfth century, Thorlogh More O'Conor, son of Ruari of the Yellow Hound, son of Hugh of the Broken Spear, was the recognised head of his race, both for valour and discretion. By some historians he is called the half-brother of Murtogh O'Brien, and it is certain that he was the faithful ally of that powerful prince. In the early stages of the recent contest between North and South, Donald of Aileach had presented himself at Rathcrogan, the residence of O'Conor, who entertained him for a fortnight, and gave him hostages; but Connaught finally sided with Munster, and thus, by a decided policy, escaped being ground to powder, as corn is ground between the mill-stones. But the nephew and successor of Murtogh was not prepared to reciprocate to Connaught the support it had rendered to Munster, but rather looked for its continuance to himself. Conor O'Brien, who became King of Munster in 1120, resisted all his life the pretensions of any house but his own to the southern half-kingdom, and against a less powerful or less politic antagonist, his energy and capacity would have been certain to prevail. The posterity of Malachy in Meath, as well as the Princes of Aileach, were equally hostile to the designs of the new aspirant. One line had given three, another seven, another twenty kings to Erin - but who had ever heard of an Ard-Righ coming out of Connaught? 'Twas so they reasoned in those days of fierce family pride, and so they acted. Yet Thorlogh, son of Ruari, son of Hugh, proved himself in the fifteen years' war, previous to his accession (1021 to 1136), more than a match for all his enemies. He had been chief of his tribe since the year 1106, and from the first had begun to lay his far forecasting plans for the sovereignty. He had espoused the cause of the house of O'Brien, and had profited by that alliance. Nor were all his thoughts given to war. He had bridged the river Suca at Ballinasloe, and the Shannon at Athlone and Shannon harbour, and the same year these works were finished (1120 or '21) he celebrated the ancient games at Tailtean, in assertion of his claim to the monarchy. His main difficulty was the stubborn pride of Munster, and the valour and enterprise of Conor O'Brien, surnamed Conor "of the fortresses." Of the years following his assertion of his title, few passed without war between those Provinces. In 1121 and 1127, Thorlogh triumphed in the south, took hostages from Lismore to Tralee, and returned home exultingly; a few years later the tide turned, and Conor O'Brien was equally victorious against him, in the heart of his own country. Thorlogh played off in the south the ancient jealousy of the Eugenian houses against the Dalcassians, and thus weakened both, to his own advantage. In the year 1126 he took Dublin and raised his son to the lordship, as Dermid of Leinster, and Thorlogh O'Brien had done formerly: marching southward he encamped in Ormond, from Lammas to St. Bridget's day, and overran Munster with his troops in all directions, taking Cork, Cashel, Ardfinnan, and Tralee. Celsus, the holy Primate of Armagh, deploring the evils of this protracted year, left his peaceful city, and spent thirteen months in the south and west, endeavouring to reconcile, and bind over to the peace, the contending kings. In these days the Irish hierarchy performed, perhaps, their highest part - that of peacemakers and preachers of good will to men. When in 1132 and '33 the tide had temporarily turned against Thorlogh, and Conor O'Brien had united Munster, Leinster, and Meath, against him, the Archbishop of Tuam performed effectually the office of mediator, preserving not only his own Province, but the whole country from the most sanguinary consequences. In the year 1130, the holy Celsus had rested from his labours, and Malachy, the illustrious friend of St. Bernard, was nominated as his successor. At the time he was absent in Munster, as the Vicar of the aged Primate, engaged in a mission of peace, when the crozier and the dying message of his predecessor were delivered to him. He returned to Armagh, where he found that Maurice, son of Donald, had been intruded as Archbishop in the interim, to this city peace, order, and unity, were not even partially restored, until two years later - A.D., 1132.

The reign of Thorlogh O'Conor over Leath Mogha, or as Ard-Righ "with opposition," is dated by the best authorities from the year 1136. He was then in his forty-eighth year, and had been chief of his tribe from the early age of eighteen. He afterwards reigned for twenty years, and as those years, and the early career of his son Roderick are full of instruction, in reference to the events which follow, we must relate them somewhat in detail. We again beg the reader to observe the consequences of the destruction of the federal bond among the Irish; how every province has found an ambitious dynasty of its own, which each contends shall be supreme; how the ambition of the great families grows insatiable as the ancient rights and customs decay; how the law of Patrick enacted in the fifth century is no longer quoted or regarded; how the law of the strong hand alone decides the quarrel of these proud, unyielding Princes.