Inis Mór is accessible year-round, but the experience varies considerably by season. Summer (June–August) brings the crowds — Dún Aonghasa can have queues in July — but also the best weather for swimming and cycling, and traditional music in the pubs most nights. The island is genuinely beautiful in summer, the wildflowers across the limestone karst are at their peak in June, and the long Atlantic evenings make for memorable sunsets.
Spring (April–May) and autumn (September–October) are excellent and less crowded. The light is good, the weather is changeable but usually manageable, and the island has a more authentic feel without the high-season tourism. May is particularly good: the spring flowers are out, the light is extraordinary, and the ferry runs reliably.
Winter visits are possible and have their appeal — the Atlantic at full force, the island to yourself, the pubs warm and quiet. But ferries can be cancelled in rough weather, and some accommodation and restaurants close between November and March. If you go in winter, have flexibility in your schedule.