The Llŷn Peninsula extends 30 miles (50 km) into the Irish Sea from North West Wales, south west of the Isle of Anglesey. It is part of the historic county of Caernarfonshire, and historic region and local authority area of Gwynedd. Much of the eastern part of the peninsula, around Criccieth, may be regarded as part of Eifionydd rather than Llŷn, although the boundary is somewhat vague. The area of Llŷn is about 400 km2 (150 sq mi), and its population is at least 20,000.

Anglesey is an island off the north-west coast of Wales. It forms the bulk of the county known as the Isle of Anglesey, which also includes Holy Island and some islets and skerries. The county borders Gwynedd across the Menai Strait to the southeast, and is otherwise surrounded by the Irish Sea. Holyhead is the largest town, and the administrative centre is Llangefni. The county is part of the preserved county of Gwynedd.