Llŷn Peninsula
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  The Llŷn Peninsula (Welsh: Penrhyn Llŷn or Pen Llŷn) extends 30 miles into the Irish Sea from north west Wales, south west of the Isle of Anglesey. It's the modern part of Gwynedd. The eas part is  of Eifionydd , although the boundary is somewhat vague. The area of Llŷn is about 400 km2(150 sq miles), and its population is at least 20,000

More details: https://en.wikipedia.org/wiki/Ll%C5%B7n_Peninsula