Oileán Cróna (Illancrona) is a small uninhabited island living one kilometre west of Termon in Maghery Bay. The island is a raised shingle bar which stretches over a kilometre running south-east to north-west and reaches a height of approximately 6 metres at its north-west end. The bar tidally comprises three separate islands with the main two being joined by a shingle bar at low to mid-tide.
The main island at the north-west of the bar has the remains of a small dwelling and outbuilding and with no soil or freshwater, the dwelling provides cover in a location where human survival would be at best very difficult indeed. The main island is a busy breeding site with the ruins of the building being full of huge ground-nesting barnacle geese in the breeding season months.
Access is by a 1-kilometre paddle from the small sheltered bay at Portaneggy just to the north of the small Irish light on the Terman Peninsula. The nearest neighbouring islands are Inishkeeragh to the north and Bristi Sea Stack to the south along the Crohy Head Coast south of Maghery.