Cellachán Caisil (‘of Cashel’) (d. 954), son of Buadachán and overking of Munster, was the last notable overking of the dynastic federation of the Éoganachta. Although Cellachán's lineage, Clann Donngaile of Éoganacht Chaisil, had provided several previous kings, his immediate line had produced no overking for several generations. His predecessor was Lorcán, whose lineage of Cenél Conaill of the Éoganachta claimed common descent from Fáilbe Flann (qv). An ambitious ruler, Cellachán sought to extend the influence of the Éoganachta beyond the province of Munster, which brought him into direct conflict with the interests of the still powerful Southern Uí Néill. The most complete account of his career is supplied by the Annals of the Four Masters. Although Middle Irish king-lists credit him with a reign of only ten years, Cellachán came to prominence in 936 when he sacked Clonmacnoise, an ecclesiastical centre that had already attracted Uí Néill support. Three years later Cellachán, now styled ‘king of Munster’, with allies including dynasts of Uí Fothaid in the eastern marches of Munster and Hiberno-Norse from Waterford, plundered Clonenagh and Killeigh and ravaged the borders of the Uí Néill overkingdom as far as Clonard. The response of Uí Néill was swift and the hostages of Munster were secured.
Subsequently, when Cellachán endeavoured to reassert his authority over the territory of the Déisi, its rulers joined forces with the king of Osraige and defeated him. The Uí Néill king of Tara, Donnchad Donn (qv) son of Flann Sinna (qv), took advantage of the situation in 941 to raid Munster and take Cellachán prisoner, detaining him for nine months. Claims by later synthetic historians that Cellachán had promoted Cennétig (qv) son of Lorcán, king of Dál Cais, to a joint provincial kingship, with the intention that he deputise for him during any such absence, fail to convince. In all probability, Cellachán's difficulty was exploited by Cennétig so that the former, on his return to Munster, faced a revolt of his sub-kings. The resistance was effectively dealt with: in 944 Cellachán defeated his challengers at the battle of Gort Rottacháin (AU) or Mag Dúin (AFM). The death (also in 944) of Donnchad Donn, king of Tara, ushered in a period of relative instability for the Uí Néill dynasties; Congalach Cnogba (qv) son of Máel-mithig eventually secured his claims to the overkingship. Taking advantage of the situation, Cellachán and his son Donnchad invaded Mide (951), plundering Clonfert, Gallen, and the territory of Delbna in Westmeath. The death of Cellachán (954) marks the effective end of the Éoganacht overkingship of Munster; almost certainly, that explains the commemoration of his achievements in the propaganda work ‘Cathréim Cellacháin’, which in essence represents a response by the rulers of south Munster to the production of ‘Cogadh Gáedhel re Gallaibh’ by the Ua Briain lineage in the north of the province.
Cellachán's immediate successors were ephemeral rulers. His son Donnchad, who claimed to be overking of Munster, was slain by his own brother in 963. The direct descendants of Cellachán took the family name of Ua Cellacháin (O'Callaghan), although it was the line of Mac Carthaig (MacCarthy), descended from Cellachán's great-grandson Carthach, that provided most of the later kings of Desmuman (south Munster).