The Diocese of Clonfert was sometimes referred to as the Bishop of Hy-Many. The see of the bishopric is located at Loughrea. The early Irish monastery and school of Clonfert, founded by Saint Brendan, brother of Saint Briga, was the dominant ecclesiastical centre in the area and an important centre of learning in the early Irish church. Cummian, an important theological writer was from there. It was also deeply involved in the eighth century spiritual reform movement of the Culdees (Céili Dé). Saint Brendan the Navigator's fame as a sea-faring missionary contributed to its pre-eminence in later times and led to its choice as an episcopal see in the twelfth century. Legend says that this community had at least three thousand monks, and that their rule was dictated to Brendan by an angel.

Like most dioceses in Ireland, the present Diocese of Clonfert had its origin in the Synod of Rathbreasail in 1110, reaching its final form at the Synod of Kells in 1152 when it was made a suffragan of the Archdiocese of Tuam. Rathbreasail was the third of four great reforming Irish synods, the other three were at Cashel (1101), Kells (1152) and Cashel again (1172).

The diocese was divided into four deaneries, Clonfert, Loughrea, Urrachree and Duniry, having 15 rectories and 39 vicarages with a chapter and offices after the Norman pattern. Four houses of Canons Regular and four of Canonesses were established in the Irish deaneries. In the 14th and 15th centuries, bishops introduced the mendicant orders: the Franciscans to Kilconnell, Kilnalahan and Meelick, with their 3rd Order to Clonkeenkerril and Kilbocht; the Dominicans to Portumna, with their 3rd Order to Kilcorban; and the Carmelites to Loughrea.

In 1170, the Annals of Clonmacnois record that "there was a great convocation of the clergy of Ireland at Clonfert by commission from the Pope for the reformation of certain abuses of a long time used in Ireland", which was presided over by Saint Laurence O'Toole presided as papal legate. Lorcán Ua Tuathail, also known as St Laurence O'Toole, was born at Castledermot (Diseart Diarmada), Kildare, Ireland, 1128, died at Eu, Normandy, France, on November 14, 1180, and was canonized in 1225 by Pope Honorius III. He was one of four sons of an O'Byrne princess and Murtagh O'Tuathail, King of the Uí Muirdeaigh. When St Laurence O'Toole was thirty-two he was elected unanimously Archbishop of Dublin following the death of Archbishop Gregory in 1161. He was the first Irishman appointed to the See of this town of Danes and Norwegians. He was the first Irishman to appointed to the See of this town of Danes and Norwegians; it is notable that his nomination was backed not only by the High King Ruaidri Ua Conchobair, Dermot McMurrough (who was now married to Lorcán's sister, Mor) and the community at Glendalough, but also the clergy and population of Dublin itself.

The family were of the Uí Dunlaigne sub-sept, the Uí Muirdeaigh, and took their surname from Tuathal mac Augaire, the Uí Muirdeaigh King of Lenister who died in 958. They resided at Maistiu or Mullaghmast in what is now County Kildare. However by the time of his son's birth Murtagh was subordinate to the new Kings of Leinster, the Uí Cheinnselaigh. The king from 1126 was Diarmait or Dermot McMurrough. At age ten he was sent to Dermot as a hostage for his father.

The Uí Dunlainge anciently inhabited the Liffey Plain, the territory around the River Liffey, just to the northwest of the Wicklow Mountains. They were very important in north Leinster, and held the provincial overkingship of Leinster itself from 738 to 1042, alternating it between their chief clans, the Uí Dunchada, Uí Faelain and Uí Muiredaig. Their representatives in later times were the O’Byrnes and O’Tooles. The Uí Faelain included the O’Byrnes (O Broin) and their kinsmen the MacKeoghs or Kehoes (Mac Eochaidh) of Leinster. The O’Byrnes descend from Bran, son of Maolmordha, King of Leinster in 1014. Maolmordha died fighting on the side of Earl Sigurd of Orkney against Brian Boru, High-King of Ireland, at the battle of Clontarf in 1014).

The O’Byrnes originally possessed what is now the northern half of County Kildare, which was called after the Uí Faelain. They were driven from this territory by the Normans, soon after the Anglo-Norman invasion, after which they retired to the fastness of the nearby Wicklow Mountains. Here they became very powerful, and at the head of the Wicklow clans they terrorized the invaders, first the Anglo-Normans, and later the English, both of whom they defeated in many a fierce engagement. Their territory in these times was known as Criochbhranach, and Included the Barony of Newcastle with parts of the baronies of Ballinacor and Arklow.

The Uí Muireadhaigh or O’Tools (O Tuathail) descend from Ughaire, King of Leinster (died 956), and were chiefs of what is now the southern half of County Kildare, which bore the designation of Uí Muireadhaigh after their clan-name. They were driven from this territory by Walter de Riddlesford soon after the Anglo-Norman invasion, afterwards retiring to the mountain fastness of Wicklow, like their O’Byrne kinsmen. Here their new territory comprised first Uí Mail on the western slope of the mountains, and later Feara Cualann, in the north. Here, in alliance with their kinsmen the O’Byrnes, they carried on incessant warfare with the invaders, Anglo-Normans and later English, which continued over more than 400 years. They maintained their independence as a clan down to the close of the reign of Elizabeth I (ca. 1600), after which the whole of Fercuolen was confiscated by the English. The O’Tooles however retained considerable property for a time, and a branch of the family settled as well in west Connacht, where they became numerous.

The story goes that when Murtagh arrived at Glendalough for Lorcan, he stated that he would draw lots to have one of his sons made a priest, at which Lorcan laughed as he had long thought of doing so. No lots were drawn, and Lorcan stayed at Glendalough. In time he rose to become Abbot of Glendalough at the age of twenty-six in 1154.

At one point Murtagh's loyalty to Dermot must have become suspect as Lorcan was imprisoned for some two years in extreme austerity, and barely given enough to live. Due to the intercession of the Abbot of Glendalough, Wicklow- members of Lorcan's family had being buried at one of its churchs for generations - relations were amicably restored between Dermot and Murtagh.

In Glendalough (Gleann Dá Locha), the ruins of several churches of the 11th and 12th centuries was part of the center of Irish Christianity that St. Kevin, a hermit priest founded in the sixth century and was destroyed in 1398 by English troops. The name Glendalough comes from the Irish Gleann Dá Locha meaning the glen (valley) of two lakes. The monastery site is located between two lakes (Lower Lake and Upper Lake). The round tower was built during the era of the viking invasions in Ireland up to and around AD 1066.

In 1166 Dermot MacMurrough was deposed as King of Lenister by an alliance of Irish Kings and Princes, led by High King Ruaidri mac Tairrdelbach Ua Conchobair and King Tighernan Ua Ruairc of Breifne; Dermot had in 1152 abducted Dervorgilla, Ua Ruairc's wife and on the death of Dermot's protector, High King Muirchertach MacLochlainn in 1166, he paid the price.

Exiled and with only a half-hearted promise of help from Henry II, after much wandering in Wales, England and France, he returned to Ireland with a group of penniless and down-on-their-luck Norman, Fleming and Welsh allies to help him regain his kingdom. The expedition succeeded beyond their wildest dreams; Dermot was reinstated as King of Lenister, the Norse towns of Wexford, Waterford and Dublin captured, and the Irish under the High King defeated. To seal the alliance, Dermot offered his daughter, Aoife — who was also Lorcan's niece — in marriage to the leader of the Normans, Strongbow.

The last years of Lorcán's life was defined by these events and those that were consequent upon it. He had been in negotiations with Dermot MacMurrough when he and his allies laid siege to Dublin after a band of Norman knights seized the town. He acted again as mediator when the King of Dublin unsuccessfully tried to recapture his town, and again when Ua Conchobair laid siege.

The arrival of Henry II in Dublin served a number of purposes: first, to rein in his erstwhile Norman subjects before they established a rival Norman kingdom of their own; second, to receive the submission of the Irish Kings and Princes; third, to arrange a Synod at Cashel. This was to bring Ireland in line with Church observances as practised in Henry's other domains in England and in France.

Two of the statutes proclaimed concerned the marriage laws of the Irish clergy, and the granting of the Rock of Cashel to the Church. The earliest and most lofty of the Cashel edifices is the round tower, a very perfectly preserved 28 metre (90 feet) high example, which dates from c.1100 and the Chapel of King Cormac, Cormac's Chapel, consecrated in 1134.