Garnish Island is an island in Glengarriff harbour, part of Bantry Bay in southwest Ireland, which is a popular tourist attraction. The Office of Public Works, which maintains the gardens on the island, uses the name Ilnacullin from the Irish Oileán an Chulinn, meaning 'island of holly'.
The garden was designed by Harold Peto, for its owner John Annan Bryce, a native of Belfast, who, with his wife Violet, purchased the island from the War Office in 1910. Violet died in 1939 and their son Roland bequeathed the island to the Irish nation in 1953. It was taken over and is still maintained by the Office of Public Works.
Renowned for its gardens and architecture, the island has played host to writers including George Bernard Shaw and George William Russell.
 
There is a restored Martello tower on the island dating from the time of the Napoleonic Wars. The Garnish tower, like others in Cork, but unlike other Irish Martello towers, has a straight cylindrical shape that does not splay out at its base.
The island of Ilnacullin extends 37 acres and is renowned for its gardens which flourish in the mild humid micro-climate of Glengarriff harbour assisted by a mainly pine shelter belt. Structures include a clock tower, a Grecian temple, a Martello Tower, and an Italian casita.