Blennerhassett was the son of Sir Arthur Blennerhassett, 3rd Baronet, whose ancestors had settled in Kerry under
Queen Elizabeth, and his wife Sarah Mahony. He was educated at
Downside School,
Stonyhurst College and
Christ Church, Oxford. He then spent time at the
Université catholique de Louvain, before travelling in
Germany. In 1849, he succeeded his father as baronet. ==Family==