For the part which Donnell O’Brien thus acted, he had to defend himself from the merited vengeance of the Irish monarch; and though he was for a time able to ward it off by the assistance of Robert Fitzstephen, he deemed it prudent, on the death of Mac Murrogh in 1171, to return to his allegiance to Roderic, and give him hostages for his fidelity. On the arrival of King Henry II. in Ireland, however, in 1172, he again submitted to the authority of the English monarch, to whom he came upon the banks of the river near Cashel, swore fealty, and became tributary.
But these oaths were not long held sacred by Donnell. The return of the king to England was soon followed by a general outburst of the Irish princes against the unjust encroachments of the adventurers, and Donnell O’Brien, once more taking possession of Limerick, led his troops, which were strengthened by the battalions of West Connaught, into the strongholds of the English in Kilkenny, who hastily retreated before them into Waterford, and left the country a prey to their devastations. To punish these daring aggressions of Donnell, Earl Strongbow, in the following year, as stated in the Annals of Inisfallen, collecting a large body of the English from the various parts of Ireland, marched into the heart of O’Brien’s territory, where he was met and encountered by him at Thurles, and defeated with a loss of four knights and seven hundred men. Strongbow, returning to Waterford, found the gates closed against him; the people, hearing of his defeat, having seized on the garrison in his absence, and put them to the sword. After a month’s sojourn on the little island, as it is called, in the mouth of the river at Waterford. Strongbow returned to Dublin, and summoning a council of the chiefs, it was determined to carry on the war with the king of Limerick with the greatest vigour. The success which they experienced might, however, have been of a different kind, if they had not been joined on this occasion by the king of Ossory, who had been already so grievously treated by O’Brien, and who was naturally rejoiced at the opportunity thus afforded him of wreaking his revenge upon his old enemy.
“With the good likeinge,” says Maurice Regan, “of all the chieftains, Reymond le Grosse, the Constable of Leinster, whoe was a man discreete and valiaunt, and by his parents of good livelyhood, was designed to be general of the army: their randevouse for the assembling of their troops was Ossory. The kyng of Ossory joined with them, and undertook to guide the army upon O’Brien. Nevertheless, Reymond mistrusted his faith, whyche the kyng of Ossory perceaving, protested his integritie with suche fervency, as it gave full satisfaction, that he would be faithfull unto him; which Donald performid with sinceritie, in guiding the army until it came to the cittie of Limericke, whyche was invironed with a foule and deepe ditch with running water, not to be passed over without boats, but at one foord onely. At the first approach the soldiers were discouraged, and mutinied to return, supposing the citie, by reason of the water, was impregnable. But that valiaunt knight, Meyler Fitz Henry, having found the foord, wyth a loud voice cried, ‘St David, companions, let us courageouslie pass this foord.’ He led the waye, and was followed but by four horsemen, who, when they were gotten over, were assailed by the enemie.”