Garinish Island, also called Ilnacullin, is the reason many people stop in Glengarriff. The island sits in the sheltered harbour and is reached by small ferry boats from the village; the crossing is short, scenic, and often includes a look at the local seal colony.
The gardens are the point: Italianate terraces, a walled garden, mature trees, and views back to the Caha Mountains. It is compact, but not a quick tick-box sight. Give it enough time to wander slowly, especially if the weather is doing that West Cork trick of changing every ten minutes.