Cellachy held the post of Chief Ollam of Ireland and died in 1079.
His obit is given in the Annals of the Four Masters as follows- "M1079.4 Ceallach Ua Ruanadha, chief poet of Ireland in his time, died."
His obituary is given in the Annals of Ulster as follows- "U1079.1 Cellach ua Ruanada, chief ollav of Ireland, rested in peace."
An Irish clavis or metrical tract has been preserved in the Book of Leinster as well as in the eighteenth-century manuscript Trinity College H 1 15, where it was copied from a fourteenth-century exemplar that seems now to be lost.
Rudolf Thurneysen incorporated this poem in his Mittelirische Verslehren as Verslehre IV.