Thomas D'Arcy McGee

The total population of Ireland, when the Normans first entered it, can only be approximated by conjecture. Supposing the whole force with which Roderick and his allies invested the Normans in Dublin, to be, as stated by a cotemporary writer, some 50,000 men, and that that force included one-fourth of all the men of the military age in the country; and further, supposing the men of military age to bear the proportion of one-fifth to the whole number of inhabitants, this would give a total population of about one million.