In the meantime domestic wars multiplied with extraordinary rapidity. Dermod Mac Murrough, the infamous King of Leinster, now appears for the first time in the history of that country which he mainly contributed to bring under the English yoke. He commenced his career of perfidy by carrying off the Abbess of Kildare from her cloister, killing 170 of the people of Kildare, who interfered to prevent this wanton and sacrilegious outrage. In 1141 he endeavoured to crush the opposers of his atrocious tyranny by a barbarous onslaught, in which he killed two nobles, put out the eyes of another, and blinded[243] seventeen chieftains of inferior rank. A fitting commencement of his career of treachery towards his unfortunate country! In 1148 a temporary peace was made by the Primate of Armagh between the northern princes, who had carried on a deadly feud; but its duration, as usual, was brief. Turlough O'Brien was deposed by Teigue in 1151. He was assisted by Turlough O'Connor and the infamous Dermod. The united armies plundered as far as Moin Môr,[244] where they encountered the Dalcassian forces returning from the plunder of Desmond. A sanguinary combat ensued, and the men of north Munster suffered a dreadful slaughter, leaving 7,000 dead upon the field of battle. This terrible sacrifice of life is attributed to the mistaken valour of the Dal-Cais, who would neither fly nor ask quarter.
In 1157 a synod was held in the Abbey of Mellifont, attended by the Bishop of Lismore, Legate of the Holy See, the Primate, and seventeen other bishops. Murtough O'Loughlin, the Monarch of Ireland, and several other kings, were also present. The principal object of this meeting was the consecration of the abbey church and the excommunication of Donough O'Melaghlin, who had become the common pest of the country. He was, as might be expected, the particular friend and ally of Dermod Mac Murrough. His last exploit was the murder of a neighbouring chief, despite the most solemn pledges. In an old translation of the Annals of Ulster, he is termed, with more force than elegance, "a cursed atheist." After his excommunication, his brother Dermod was made King of Meath, in his place.
At this synod several rich gifts were made to the abbey. O'Carroll, Prince of Oriel, presented sixty ounces of gold. O'Loughlin made a grant of lands, gave one hundred and forty cows and sixty ounces of gold. The Lady Dervorgil gave the same donation in gold, together with a golden chalice for the altar of Mary, with gifts for each of the other nine altars of the church. Dervorgil was the wife of Tiernan O'Rourke, Lord of Breffni, who had been dispossessed of his territories in 1152; at the same time she was carried off by Dermod Mac Murrough. Her abduction seems to have been effected with her own consent, as she carried off the cattle which had formed her dowry. Her husband, it would appear, had treated her harshly. Eventually she retired to the Monastery of Mellifont, where she endeavoured to atone for her past misconduct by a life of penance.