There were differences in practices between the two traditions, which caused many disputes and these came to a head at the Synod of Whitby, in 664 AD.

A decision on the supremacy of Rome as opposed to the Celtic monastic traditions and the method of calculating the date of Easter were taken in favour of the English/Roman church.

Colman, who had pleaded the Celtic cause, resigned as Bishop of Lindisfarne, and he and thirty Saxon monks and the large body of Irish monks living on Lindisfarne retreated to Iona.

They spent two years on the island of Iona in prayer and contemplation following which they set sail for Ireland. They founded a monastery on the island of Inishbofin, off the coast of County Galway. Disputes arose between the Saxon and Irish monks after a short time.

Colman brought his Saxon followers onto the mainland and founded a monastery for them at "Magh Eó" - the Plain of Yew Trees. They were joined here by Gerald and a large community of monks who were living at nearby Rosslee. Gerald was the son of a Saxon prince and a follower of Colman.

He, his three brothers and a large group of Picts had left Northumbria following the Synod at Whitby and eventually came to Rosslee, which is in the west of the modern parish of Mayo Abbey, where they founded a monastery. The community was ravaged by plague and many died. They moved to Magh Eó and Gerald was appointed as first abbot of the newly founded monastery. Colman returned to Inishbofin where he died some years later.

The Monastery at Magh Eo grew quickly. The monastic enclosure covered more than 28 acres and Gerald and his monks were endowed with grants of land which eventually amounted to more than 2,000 acres.

By the year 700 AD. the monastery had become a famous seat of learning with more than one hundred monks living there. It became known throughout Christendom as "Mayo of the Saxons".

For several centuries it remained a Saxon establishment and there are many recorded contacts between it and Iona and Northumbria. Adomnán, the ninth abbot of Iona and biographer of Columba, is reputed to have lived in Magh Eó between the years 797 to 802 AD. Alcuin of York, chief advisor to the Emperor Charlemagne, corresponded with the monks at Magh Eó on several occasions.

The importance and size of the monastery is recorded in many chronicles of that period, the most important being the "Ecclesiastical History of the English People", written by The Venerable Bede of Jarrow and The Annals of Ulster.