The Annals of the Four Masters of Ireland tells us that in 1213 he was living in Cairbre Drom Cliabh, now County Sligo as the ollamh (high poet) of King Domhnall Ó Domhnaill (died 1241) of the O'Donnell dynasty.
His life took a drastic turn after he killed his benefactor's tax-collector Fionn Ó Brolacháin, whom Muireadhach considered had been insolent, with an axe.
In a poem, Ó Dálaigh dismisses his murder as his victim was a mere commoner and therefore of no account, a telling indication of the rigid stratification of traditional Irish society: Trifling is our difference with the man, A shepherd was affronting me; And I killed that clown; O God!
[2] He first placed himself under the protection of Richard Fitzwilliam de Burgh, member of a Norman family resident in Connacht for thirty years, to whom he addressed the poem in praise of his beauty and his adoption of Gaelic culture, "Créd agaibh aoidhigh a gcéin" ("Whence comes it that you have guests from afar?").
He wrote a poignant poem on the death of his wife, who had borne him eleven children in twenty years: "M'anam do sgar riomsa araoir" ("My soul parted from me last night").
He passed through Ireland on his return, where he wrote a poem to Murchad Ó Briain, a descendant of Brian Boru, praising his ancestry.