Mona, now known as Anglesey in Wales, was storied to be the home of the Druids as mentioned in accounts by Julius Ceaser. In AD 60, the Romans under Suetonius Paullinus, decided that it was vital to invade Anglesey and destroy the power of the Druids. The Romans eventually won the battle and destroyed the Druids on Anglesey and cut down their groves of sacred oaks.
Entrance to Unstan Chambered Cairn, Orkney, Scotland; dates from 4000 BC; this type of cairn often had stone slabs dividing the cairn into at least 4 chambers, sometimes with forecourts, and were probably in use over a long period of time - Wikipedia
From cromlechs to cairns - ancient burial sites in Wales
Bryn Celli Ddu on Anglesey is one of the finest Late Neolithic passage graves in Wales and is aligned to the midsummer sunrise. Inside is an unusual upright stone, carved with spirals which now resides in National Museum Wales in Cardiff
Ballymacdermott Cairn, County Armagh, Northern Ireland - This monument, lying on the southern slopes of Ballymacdermot Mountain close to Newry, is an extremely well preserved Neolithic burial site with three chambers. It can be dated between 4000 and 2500 BC.