From Fr. Walsh's History of the Irish Hierarchy:
St. Nessan, who died AD 552, was the disciple of St. Ailbe of Emly. Nessan was most probably the founder of Mungret monastery. He never attained in the church a higher dignity than that of deacon by which title he is constantly known, yet his reputation is so great that he is reckoned among the fathers of the Irish church. His festival was celebrated at Mungret on the 25th of July.
The monastery of the island called Ireland's Eye in the county of Dublin is said to have been founded by a St. Nessan. That isle originally called Inis faithlen obtained the name of Inis Mac Nessan from the sons of Nessan, who were revered there on the 15th of March. Their names are Dichull Munissa and Neslug.
A.D. 552 died the abbot St. Nessan. St. Manchin, called the first bishop of Limerick, is said to have been abbot of Mungret.
A.D. 760 died the abbot Ailill.
A.D. 820 and 834 the Danes devastated the abbey of Mungret.
A.D. 1102 the 5th of October, the blessed Mugron O'Moore, principal professor of divinity at Armagh and in all the west of Europe, died in this abbey.
A.D. 1107 this abbey was again sacked. The psalter of Cashel relates that this establishment had at one period six churches within its walls and contained, exclusively of scholars, 1500 religious, 500 of whom were learned preachers, 500 were psalmists, and the remaining 500 applied themselves to contemplation, works of charity, and other spiritual exercises.