Ua Cerbhaill was a successful political operator on both a regional and local level during the period of instability associated with "High Kings with Opposition".
Early in his career, from his Clann Cearbhaill base in Fearmaigh and west Louth, Ua Cerbhaill conquered the sub-kingdom of Conaille Muirtheimne (c. 1130) in north Louth, raided across Meath into Fingal, the Viking settlement north of Dublin, and closely cooperated with the northern power of Muirchertach Mac Lochlainn in his actions in Ulaid and Midhe.
This enabled him to take the Midhe sub-kingdom of Fir Arda Ciannachta into his kingdom (by c. 1140), and extend his influence around Newry and the area of Bairrche (the Mournes) in south County Down.
Their Mhigh Eotach or Mac Eochy clan gave their name to Ballymageogh townland and Slievemageogh a mountain in the Mourne district.
[1] Donnchad Ua Cerbaill's religious foundations show his success in tying the reform movements interests to his own, facilitating as they did the consolidation of conquests he made.
In alienating older religious sites from local dynasts, who traditionally held them hereditarily, to European institutional orders he undermined their use as a power base for future opposition.