Robert Blennerhassett (c.1622 - c.1689) was an Anglo-Irish soldier and politician.
Blennerhassett was the son of Captain John Blennerhassett and Martha Lynn. He was probably born at the family estate at Ballycarty Castle, County Kerry. The Blennerhassett family, originally from East Anglia, had been granted large estates in Ireland by the Crown as part of the Plantations. He was the brother of John Blennerhassett MP.
Blennerhassett served as an officer in the Cromwellian army during the Irish Confederate Wars, ensuring that his lands remained intact. He was granted a full pardon following the Restoration of the Monarchy. He served as the Member of Parliament for Tralee in 1674, as had his grandfather. Blennerhassett then served as High Sheriff of Kerry in 1682.
He married Avice Conway, the daughter and heiress of Edward Conway, and a descendant of Henry I. In doing so he came into the possession of Castle Conway in Killorglin, which would subsequently become the manor of the Blennerhassett's 7,000 acre Kerry estate. He had eleven children with Avice Conway. He was the grandfather of Conway Blennerhassett, and the great-grandfather of William Spring, Harman Blennerhassett and Sir Rowland Blennerhassett, 1st Baronet. His wife drowned at sea in 1683, and the last recording of Robert Blennerhassett is in 1689.