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.