Seachnasach mac Donnchadh () 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 23rd 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.
Pedigree Seachnasach mac Donnchadh mc. Comahaighe m. Fergal m. Maolciarain m. Maoltuile m. Siodhuine m. Nocba m. Egma m. Gabhnan m. Tobath m. Branan m. Brian m. Murchadh m. Aodh m. Artgail m. Guaire Aidne.
Descendants Sir Roger O'Shaughnessy of Kinelea, 2nd Baronet O'Shaughnessy of Kinelea Roger O'Shaughnessy, Captain in the Army of James II of England William O'Shaughnessy, Major-General and Chief of the Name Arthur O'Shaughnessy, poet Baron Shaughnessy, title in the Peerage of the United Kingdom Eileen O'Shaughnessy, wife of George Orwell James O'Shaughnessy, author, investor
References The Surnames of Ireland, Edward MacLysaght, Dublin, 1978, p. 52 Irish Kings and High Kings, Francis John Byrne, 2001 (second edition) The Great Book of Irish Genealogies, 257.11, pp. 586-87, volume two, Dubhaltach MacFhirbhisigh; edited, with translation and indices by Nollaig Ó Muraíle, 2003–04; Ordnance Survey Letters Galway 1838 and 1839, p. 194, ed. Michael Herity, MRIA, FourMasters Press, Dublin, 2009;