[SunkenCiv]

http://www.achill-island.com/hist.htm

"Archaeological evidence suggests that the Achill/Curraun area was densely populated 5,000 years ago. Megalithic and crannóg remains show evidence of this. The earliest settlers were food gatherers who inhabited coastal settlements."

[Irish_Thatcherite]

Slightly further north, there is the Ceide Fields, also dating back 5,000 years:

http://homepages.iol.ie/~stmrysba/ceide2.htm