Based on certain linguistic characteristics and some historical references, the earliest life of Fintán/Munna is believed to date around the year 800.
"No" answered Baithen, "but, although I should be very glad to keep you with me, I must obey the orders of my predecessor Columba, who some time ago said to me in the spirit of prophecy; Baithen remember these words of mine; immediately after my departure from this life, a brother, who is now regulating his youthful age by good conduct, and well versed in sacred studies, named Fintan, of the race of MocuMoie, (74) and son of Failchan, will come to you from Ireland (Scotia) and will supplicate to be reckoned among the monks.
Do not therefore let him remain in these islands of ours, but direct him to return in peace to Ireland (Scotia), - that he may there establish a monastery in a part of Leinster not far from the sea, and labour for the good of souls.
Fintán was versed in scientific knowledge and gave frequent public lectures where Christian revelation was illuminated by the sciences and mathematics.
[1] He did have an abbey in Heli (or Hele, perhaps in County Westmeath), which he and his monks forsook, giving it to Saint Cera and her companions.
Saint Cera named the place, according to Fintán's instructions, after St. Telle who had given three jubilations in the plain of Miodhluachra that day.
[5] Toward the end of his life, he opposed the adoption of the Roman method for determining the date of Easter in opposition to Laserian.
[16] Munnu withdrew his opposition and adopted the Roman system with the rest of southern Ireland.