Oileán na d'Trachtá is a very small uninhabited island linked by a grass covered causeway to the bridge/causeway, which links the islands of Lettermore and Annaghvaan. I visited at dusk, so didn't stay long. There was a faint but rather overgrown path on to the island. The island is 250 metres from north to south by 200 metres from east to west.
Oileán na d'Trachtá from Lettermore
Looking south towards Lettermore down the grassy causeway