The Llŷn Peninsula is a peninsula in Gwynedd, Wales, with an area of about 400 km2, and a population of at least 20,000. It extends 30 miles into the Irish Sea, and its southern coast is the northern boundary of the Tremadog Bay inlet of Cardigan Bay. The peninsula was a cantref within the medieval kingdom of Gwynedd, and became part of Caernarfonshire from 1284 until that county was abolished for administrative purposes in 1974.