Cambrensis notes that ‘his youth and inexperience in government led him to become the oppressor of the nobility.’ His education was entrusted to Aedh mac Crimthainn, abbot of Terryglass, co. Tipperary, termed ‘the chief historian of Leinster,’ for whom the ‘Book of Leinster’ is said to have been compiled by Bishop Finn of Kildare, who was previously abbot of Newry. Dermod appears to have profited little by his instruction. Cruelty and profligacy characterised his youth. He is described by Giraldus as of giant stature, his voice hoarse from shouting his war-cry in battle, his hand against every man and every man's hand against him. According to the ‘Chronicon Scotorum,’ at the age of twenty-two he forcibly abducted the Abbess of Kildare, and when the community endeavoured to prevent the crime he slew 140 of them and set fire to the monastery.
In the confusion which prevailed in the government of Ireland at this period, Dermod asserted a claim to the whole south of Ireland, called Leth Mogha. Accordingly he invaded Ossory in 1134, and though repulsed at first he returned to the attack and defeated the people of Ossory and their allies the Danes of Waterford. In 1137 he besieged Waterford, which was within the territory he claimed. In 1149 he plundered the stone-church of St. Cianan of Meath with the assistance of the Danes. Laurence O'Toole, then a boy of ten, was delivered into his hands, and was treated by him with such cruelty that O'Toole's father threatened to execute twelve of Dermod's followers unless the boy was restored to him. He is further charged in the ‘Annals of the Four Masters’ with putting to death or depriving of sight seventeen of his subordinate chieftains, though Leland attributes this offence to his father. The crime for which he is chiefly notorious was the abduction of Dervorgill, wife of Tiernan O'Ruark, lord of Breifne, a territory comprising the counties of Leitrim, Longford, and Cavan. The Anglo-Norman writers and the native annals supply different versions of the affair. The former, of whom Giraldus Cambrensis is the principal, describe Dervorgill as taking advantage of her husband's absence to invite Dermod to carry her off, and as feigning reluctance. Keating, who follows Giraldus, adds that her husband was at the time on a pilgrimage to St. Patrick's Purgatory at Lough Derg, and both writers agree that Dermod was expelled from his kingdom for this act, and that his journey to England and the Anglo-Norman invasion were the immediate consequences of it. But according to the more probable account in ‘Annals of the Four Masters’ under the year 1152 it was when the combined armies of O'Connor, Dermod, and others had invaded O'Ruark's territory, defeated him and deprived him of the district of Conmaicne, that Dermod took the opportunity of ‘carrying off Dervorgill with her cattle and furniture,’ whether with or without her consent is not stated. In the following year O'Connor, who had previously been Dermod's ally, marched against him, retook Dervorgill, and delivered her to her kinsmen the people of Meath. In the course of the same year she, according to the ‘Four Masters,’ ‘came to her husband again.’ In 1157 she was present with her husband at the consecration of the church of Mellifont, co. Louth. She survived her husband twenty-one years, and died in the monastery of Mellifont in her eighty-fifth year, in 1193.
Meanwhile political changes were going forward; O'Loughlin, who had been Dermod's ally, was killed in the battle of Litterluin in 1166, whereupon Roderick O'Connor his enemy became king of Ireland, and Dermod, anticipating an attack, burnt his town of Ferns. Soon after another of Dermod's enemies, O'Ruark, marched against him, defeated him, burnt the castle of Ferns, and ‘banished him over sea.’ This took place, according to the ‘Four Masters,’ in 1166, and as this was fourteen years after the carrying off of Dervorgill it is evident that there is little direct connection between the two events. It was probably the fact of his evil life that led to his liberality in founding monasteries; among these was the convent of St. Mary de Hogges for Augustinian nuns, established in 1146. To this he subjected Kilclehin in the county of Kilkenny, and Aghade in the county of Carlow. In the same year convents at Baltinglass and Ferns were founded by him, and lastly the priory of All Saints, Hoggin Green, Dublin, where Trinity College now stands, in 1166. This liberality gained him the favour of the clergy.
When banished over sea Dermod sought the aid of Henry II to recover his kingdom, imploring his protection and promising, if successful, to hold his kingdom as Henry's vassal. The application was highly acceptable to Henry, who in 1154 or 1155 had in view an expedition to Ireland, and according to many authors, obtained a bull from Adrian IV authorising the invasion, the pope sending him at the same time a valuable ring as a token of investiture. But the queen-mother being opposed to the enterprise, and matters not being ripe for action, the bull was kept secret for some years. Attempts are made from time to time to question the authenticity of this bull, but without sufficient reason. It is attested by abundant contem-
