Seachnasach mac Donnchadh

Seachnasach mac Donnchadh, ancestor to the family of O'Shaughnessy, fl. 10th century.

Seachnasach was a member of the Uí Fiachrach Aidhne dynasty, formerly Kings of Connacht. By the tenth century their power had been reduced to Aidhne, a small kingdom in what is now south County Galway. Seachnasach was a seventeen-time great-grandson of Guaire Aidne mac Colmáin, one of the dynastys most notable rulers. He was a kinsman of other Uí Fiachrach dynasts such as Giolla Ceallaigh mac Comhaltan, Scannlán mac Fearghal, Eidhean mac Cléireach, and Cathal mac Ógán, all of whom would have descendants who derived their surname from them.