Fedelmid was of the Cenél Fíngin sept of the
Eóganacht Chaisil branch of the
Eóganachta, and he is noted as having assumed the sovereignty of Munster in 820. In 823, in co-operation with Bishop Artrí mac Conchobar of
Armagh, he had the "
Law of St. Patrick" established in Munster, and sacked the monastery, that of Gailline of the Britons, in modern County Offaly. The
Dealbhna Breatha was burnt by Fedelmid in 825. In 827, there was the first of a number of royal meetings between Fedelmid and
Conchobar mac Donnchada, of the
Southern Uí Néill, King of Tara or
High King of Ireland. In 830, Fedelmid was back burning monasteries—this time it was probably that of
Fore Abbey in modern
County Westmeath, while in Southern Galway, he destroyed the
Uí Briúin, and in the same year, the Munstermen were recorded as killing Folloman, son of Donnchad, brother of Conchobhar, the High King of Ireland. In 831 and 832, he is recorded as taking an army of Leinster and Munster into East Meath, plundering as far north as
Slane, while also raiding the Dealbhna Beatha of southern
Offaly three times, and burning
Clonmacnoise. In 833, he is back in Clonmacnoise burning it and the
Clann Cholmáin monastery of
Durrow to the doors of their churches. In 835, the Munstermen are recorded as having slain Fergus, son of Bodbchad, the King of
Carraic-Brachaidhe, from the very north-west of the country, in
Inishowen. ==High point of his rule==