St. Gerald was an English monk, and the bishop of Mayo. The date of his birth is unknown, however we do know that after the Synod of Whitby in 664, he followed St. Colman to Ireland, and settled in Innisboffin, in 668.
After some time, dissensions arose between the Irish and the English monks, and St. Colman decided to found a separate monastery for the thirty English brethren. Thus arose the Abbey of Mayo (Magh Eo, the yew plain), known as "Mayo of the Saxons," with St. Gerald as the first abbot in 670.
St. Bede writes: "This monastery is to this day, 731, occupied by English monks...and contains an exemplary body who gathered there from England, and live by the labour of their own hands (after the manner of the early Fathers), under a rule and canonical abbot, leading chaste and single lives."
Although St. Gerald was a comparatively young man, he proved to be a wise ruler, and governed Mayo until 697, when, it is said, he resigned in favour of St. Adamnan. Some authors hold that St. Adamnan celebrated the Roman Easter at Mayo in 703, and then went to Skreen, in Hy Fiachrach, and that after his departure the monks prevailed on St. Gerald to resume the abbacy. Mayo, though merged in Tuam for a time, remained a separate see until 1579.
The Saxon saint continued to govern the Abbey and Diocese of Mayo till his death on died March 13, 731. His feast is celebrated on December 5.
Source: The Catholic Encyclopedia