Over king of Munster from 820 to 847, Fedelmid mac Crimthainn’s birth year is given in the annals as 770, although the date may have been a later interpolated entry. He was a member of the Eoganacht Chaisil branch of the Eoganachta, although he was not from a dominant segment. The last of his ancestors to hold the kingship of Munster had been Fingen mac Aedo (+ 619), and Fedelmid’s accession was unusual in a time when kingly succession was determined by relationship to a recent king, that is a son, grandson, or brother. He may have been a compromise choice at a time when Munster had been under attack by the Uf Neill kingship of Tara and needed a strong warrior as its king. He was closely associated with the Cele De church reform movement that began in the eighth century in Ireland and had much in common with the Carolingian reform associated with Benedict of Aniane. The Celi De were ascetics who disapproved strongly of the worldly state of the church in Ireland, particularly the great monasteries that were patronized by the great kings and nobles of Ireland, who often stored their wealth in stone buildings provided for that purpose. As monasteries/ecclesiastical settlements became more associated with secular interests they became vulnerable to attack, but Fedelmid was the first great Irish king to introduce the practice. It is possible that his attacks on ecclesiastical settlements such as Clonmacnoise were inspired by a puritan zeal to cleanse the old and sinful ways of the unreformed church. However, it is more likely that such attacks were part of his attempts to replace the Uf Neill as kings of Tara.
In 823, Fedelmid proclaimed the law of Patrick in Munster, with Artri mac Conchobair his favored candidate for bishop of Armagh. In the same year he commenced a war of attrition against ecclesiastical settlements in Uf Neill or border territories when he burned the monastic site of Gallen (County Offaly). In 826, he burned Delbna Bethra (in western Mide) with an army from Munster. In 827, he met the Uf Neill king of Tara, Conchobar, at Birr for what the annals called a "royal conference." Any agreement made at this meeting was short-lived as in 830, Fedelmid was recorded as inflicting a defeat on the Connachta and the Uf Neill. In 831, he plundered the territory of Conchobar near Slane, and the latter replied by plundering the Liffey plain. In 823, he burned the church lands of Clonmacnoise "to the very door of the church" and put to death members of the community. He did likewise at Durrow. He burned the termon (sanctuary lands) of Clonmacnoise in 832 and plundered the surrounding land in Delbna Bethra three times in that same year. His first reverse came from Cathal, son of Ailell, king of Uf Maine, at Mag nAf (County Roscommon) in 835.
In 835, Fergus, son of Bodbchad, king of Carraic Brachaide, was killed by Munstermen and in 836, Dunlang, son of Cathusach, abbot of Cork, is recorded as having died without communion in Cashel of the kings. The Annals of Inisfallen do not record the death of Dunlang but mention the entry of Fedelmid into the abbacy of Cork the same year. The foundation of Cork had been involved in several battles with other ecclesiastical settlements in that period. He attacked Uf Maine in Connacht in 837. In 838, the annals report "a great royal conference in Cluain Chonaire Tomar, between Fedelmid and Niall [Caille]," that is, at Cloncurry in County Kildare. Whatever the outcome of his meeting with Niall, Fedelmid attacked Mide and Brega—the lands of the Southern Uf Neill—the following year. He also ravaged the kingdom of Delbna Bethra as well as the neighboring Uf Neill kingdom of Cenel Fiachrach (Fir Chell in the annals). The southern annals record that in 840, he harried the north from Birr to Tara and, in 841, Fedelmid led an army to Carman. The king of Tara, Niall Caille, marched against him and defeated the forces of Munster at Mag Ochtar. Following mac Crimthainn’s defeat at the hands of Niall Caille, the Uf Neill were again firmly in place as the dominant kingship in Ireland. However, he attacked Clonmacnoise again in 846.
He died in 847 and is described in the annals as "king of Munster, a scribe and anchorite and the best of the Irish." When Fedelmid mac Crimthainn succeeded to the kingship of Munster it was in a weakened state, as his predecessor according to the regnal lists, Tnuthgal mac Donngail of Eoganacht Glenamain, is not mentioned in the annals. In fact, the annals call Artrf mac Cathail, also of Eoganacht Glenamain, who died in 821, the year after Fedelmid became king, king of Munster. The decline of the Eoganacht Locha Lein, whose over kingdom of Iarmumu collapsed when the subject peoples of west Munster transferred their direct allegiance to Cashel around the end of the eighth century, made Fedelmid’s position in Munster secure and allowed him to move beyond the province in order to take on the ever-increasing threat of the Uf Neill.
Fedelmid mac Crimthainn was an ecclesiastic, although it is not certain that he was a bishop. The phenomenon of kings who were also ecclesiastics seems to be unique to the south of Ireland, and it is possible that Fedelmid may have inaugurated the tradition of ecclesiastical kingship. His career can best be explained in the light of increasing aggression from the Uf Neill toward Munster and his membership of the Celi De, which colored his view toward the great monasteries. It could be said of him that he was the last great Eoganacht king of Cashel, as the Eoganachta dynasties began to go into decline from that time. At a time when Ireland was under attack by the Vikings it is remarkable to note that Fedelmid never struck a blow against them. Fedelmid mac Crimthainn was later revered as a saint and his feast day was celebrated on August 28, according to the Martyrology of Donegal.
