Loarn mac Eirc was a legendary king of Dál Riata who may have lived in the 5th century.
The Duan Albanach and the Senchus Fer n-Alban and other genealogies name Loarn's father as Erc son of Eochaid Muinremuir. Loarn appears in Irish traditions as 'King of Alba' in the eighth- to twelfth-century tale "Of The Miracles of Cairnech Here" in the Lebhor Bretnach, the Irish version of the Historia Brittonum, and in the tenth- to twelfth-century legendary and mythological tale Aided Muirchertach mac Erca. In these tales, mac Erca spends time with Loarn, his uncle, before murdering him by setting him aflame. Loarns main significance is as the eponymous ancestor of Cenél Loairn, a kindred whose name is preserved in Lorne.
The descendants of Loarn, the Cenél Loairn, controlled parts of northern Argyll around the Firth of Lorne, most probably centred in Lorne but perhaps including the islands of Mull and Colonsay, Morvern and Ardnamurchan. The boundary to the east was the Druim Alban mountain ridge that separated Dál Riata from Pictland. The chief places of the kingdom appears to have been at Dun Ollaigh, near Oban and Dunadd near Crinan. The chief religious site may have been on Lismore, later the seat of the High Medieval bishop of Argyll.
Several kings of Dál Riata were members of the Cenél Loairn, and thus claimed descent from Loarn.
In High Medieval times the Mormaers of Moray claimed descent from Loarn.