Background
Hennessy was born at Castle Gregory, County Kerry.
Hennessy was born at Castle Gregory, County Kerry.
After school education he emigrated to the United States, where he lived for some years. He returned to Ireland and wrote in newspapers, but concentrated on Irish literature: he was a native speaker. In 1868 he obtained an appointment in the Public Record Office, Dublin.
He rose to be the assistant-deputy-keeper, and held office till his death.
He died at his residence, 71 Pembroke Road, Dublin, 13 January 1889.