This then was the political and social structure when Eochaidh Muigh Meadhoin (OH-he Muee Moyvone) became Ard-Rí, the 124th Monarch of Ireland, in 357 AD. He had four sons, Brian, Fiachra, Oliol and Fergus, by his first wife Mong Fionn, and one son, Niall, by his second wife, Carthan Cais Dubh. From his sons sprang the powerful Uí Néill (EE-NALE), Uí Bríuín (EE-BREEN), and Uí Fiachrach (FEE-kra) line of kings of Ireland, Ulster, Midhe, and Connacht for the next 700 years. From Fiachra and Brian were descended the two most dominant dynasties of Connacht, the Uí (EE) Fiachrach and Uí Bríuín Dynasties. Fiachra's descendants gave their name to Tír-Fiachra (Cheer-FEE-krah) in north east Connacht, known today as Tireragh (CHEER-rah) in County Sligo but also included back then parts of north-east County Mayo.

Niall, Eochaidh Muigh Meadhoin's son by his second wife, was himself a powerful prince of the Connachta and became the 126th Monarch of Ireland in 378 AD. He conquered the territories of the Middle Kingdom of Brega and established next to it the Kingdom of Mida. He won fame and power by successful raids on Britain. Nine tuatha around the northern capital of Emhain Macha (EV-n MOK-ha) put themselves under the protection of Niall and formed a federation called the Airgialla (eerGEE-lah) -'the hostage-givers' - from whom Niall got the epiteth of Noígiallach (Nine Hostages) - Niall of the Nine Hostages. His descendants took the dynastic name of Uí Néill. Two of his sons, Eoghan (Owen) and Conall conquered north-west Ulster and founded there the great Northern Uí Néill Dynasty with its capital at Aileach, and the others ruled in Midhe and Brega as the equally powerful Southern Uí Néill Dynasty. The over-kings of the Uí Néill called themselves 'kings of Tara', but unlike the earlier great kings of Tara they themselves lived more modestly in raths (ring-forts) or lake dwellings such as the crannog at Lagore in County Meath, the home of the Uí Néill kings of south Brega. Almost without interruption Niall's descendants were considered the high kings of Ireland for 600 years with the position alternating between the Northern and Southern Uí Néill Dynasties.

While Fiachra's son, Dathi (DAH-hee), succeeded Niall of the Nine Hostages in 405 as Ard-Rí (High King), It was Niall's son, Laeghaire (Lah-HEE-reh), as the 128 Monarch of Ireland who received St. Patrick at Tara in 432, an event that led to the conversion of Ireland to Christianity. Patrick initially considered himself a Roman citizen and the church which he established was Roman in character and organisation, with its bishops exercising authority within their own dioceses which corresponded to the political structure of the tuatha. He chose Ard Macha (Armagh) close to Emhain Macha, the great hill fort once occupied by the Gaelic kings of Ulster, as the ecclesiastical capital of Ireland. As the gospel spread, more and more people wanted to dedicate their lives to God. In this rural society there were no great towns or cities where they could join together in prayer and contemplation, and so monasteries quickly came into existence

The collapse of the Roman Empire meant that communication with the mother church was impaired, and the Celtic Church in Ireland developed its own separate character and rites. While a number of monasteries existed before then, the first Irish monastery to become famous was founded by St Enda on the Aran Islands at the end of the fifth century. St Finnian founded Clonard early in the sixth century. He became known as the 'teacher of the saints of Ireland', for twelve of his pupils, often referred to as the 'twelve apostles of Ireland', founded a number of important monasteries. They included ones by St Columcille, or St Columba the name under which he usually appears in accounts written outside Ireland, at Derry, Durrow, Kells and at 38 other places, all by the time he had reached the age of forty-one. In fact by the time of Columcille's death in May of 597, sixty monastic communities had been founded in his name in Scotland alone.