Seachnasach mac Donnchadh

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Seachnasach mac Donnchadh (fl. 10th century) was an ancestor to the family of O'Shaughnessy.

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 dynasty's 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.