Complex sentence: He was succeeded by his eldest son Sir Leonard Blennerhassett (died 1639), who was in turn succeeded by his son Henry, who was MP for Fermanagh in 1664, and High sheriff for the county.
Split into simpler sentences: He was succeeded by his eldest son Sir Leonard Blennerhassett (died 1639). The castle subsequently returned to Blennerhassett hands, to Henry, son of Sir Leonard, who became MP for Fermanagh in 1664, and High sheriff for the county.