Ilnacullin, Private garden island in Glengarriff Bay, Ireland.

Ilnacullin is a private island in Glengarriff Bay featuring gardens spread across approximately 15 hectares with formal pathways, water features, and terraced levels. Architectural structures dotting the grounds include a clock tower, a Grecian temple, and a small Italian-style house.

Belfast-born John Annan Bryce bought the island from the War Office in 1910, when it was a bare rock with thin peaty soil. Between 1911 and 1914 he and architect Harold Peto created the gardens, importing humus from the mainland and planting a pine shelter belt. Bryce's son Roland gave the island to the Irish nation in 1953, and the Office of Public Works has maintained it since.

The Irish name Ilnacullin (Oileán an Chulinn) means 'island of the holly', while Garinis is translated as 'near island'; the holly name is thought to be the older one. The gardens were laid out between 1911 and 1914 by Harold Peto for John Annan Bryce, and visitors follow a circuit through themed areas such as the Italian garden, the Greek temple and the Jungle.

The island is accessible by ferry from Glengarriff village only during the opening season from March to October, with boat trips taking just a few minutes. Wear sturdy footwear since the terrain is hilly and weather can change quickly throughout the day.

A Martello tower on the island has an unusual straight cylindrical shape rather than the typical splayed base seen elsewhere in Ireland. This difference makes it a rare example of this military structure.