Ó Seachnasaigh, O'Shaughnessy, collectively Uí Sheachnasaigh, clan name Cinél nAedha na hEchtghe, is a family surname of Irish origin. The name is found primarily in County Galway and County Limerick. Their name derives from Seachnasach mac Donnchadh, a 10th-century member of the Uí Fiachrach Aidhne, which the Ó Seachnasaigh were the senior clan of. The town of Gort, Ireland, was the main residence of the family since at least the time of their ancestor, King Guaire Aidne mac Colmáin.
Up until the late 17th century the Ó Seachnasaighs held the sub-district of Uí Fiachrach Aidhne known as Cenél Áeda na hEchtge (modern Irish, Cinéal nAedha na hEchtghe), meaning "kindred of Aedh of the Slieve Aughty", which was also their clan name. Cinéal nAedha na hEchtghe / Kinelea consisted roughly of the civil parishes of Beagh, Kilmacduagh and Kiltartan and also parts of the civil parishes of Kibeacanty and Kilthomas. Their closest related kinsmen were the Ó Cathail / O Cahill clan, originally chiefs of eastern Kinelea, and the other clans of Uí Fhiachrach Aidhne the most prominent of which were the Ó hEidhin / O Hynes, Ó Cléirigh / O Cleary and Mac Giolla Cheallaigh / Kilkelly septs. Up until the mid-17th century, the O'Hynes clan were still styled lords of Uí Fhiachrach Aidhne even though the Ó Seachnasaighs had become more powerful than their kinsmen during this century. In the 1690s, Captain Roger O'Shaughnessy had his lands confiscated for supporting the Jacobite cause against William of Orange, with the lands going to Sir Thomas Prendergast, 1st Baronet. A legal battle raged on into the first half of the 18th century between the Ó Seachnasaigh and the Prendergasts, the family who were granted the lands, with the Ó Seachnasaigh eventually losing the case. The senior line of the Ó Seachnasaighs may have died out in the 1900s.
The last de facto Ó Seachnasaigh lord of Kinelea died at Gort after returning home from the Battle of Aughrim on 12 July 1691. Thousands of bearers of the name still survive both in their homeland and further abroad.
