The Belfast Telegraph (see www.belfasttelegraph.co.uk/news/local-national/northern-reland/maghera-tomb/ ) … Portal Tombs, a form of dolmen, are only found in Ireland, Wales, and Cornwall, and there has been an archaeological debate on their origins as they seem to appear at the same time as large rectangular long houses. They are thought to date back to around 4000BC, or a little earlier, and surprisingly very few of them have been investigated by archaeologists with modern methodologies (including the use of C14 as a dating technique). They are simply preserved as a feature of the past – something to poke around in when the funds are available (but never are). In this instance, the poking around is going to happen very soon as the capstone has slid off its support stones and revealed a square chamber beneath – untouched by grubby hands. Plans are afoot to re-erect the massive capstone and explore the interior. The findings, hopefully, will give some clues to where these migrants came from.
Note … the spread of farming communities through Europe appears to have involved a series of steps, and one such movement was associated with the aftermath of the low growth tree ring event Mike Baillie assigns to 4200BC (A Slice Through Time Batsford: 1995). On the continent this event appears to have triggered some long distance movement of people.