Ballygraffan Portal Tomb is a Neolithic portal tomb. Though dating if often widely inaccuratee it is thought to date from 4000-2500BC.
This is not a typical dolmen, since its capstone is not balanced on top of orthostats but rather just sits upon a pile of large stones. It is encircled by a drystone wall. Some sources say this is not a typical dolmen because it has collpased in on itself and other sources have suggested that there are too many stones involved here for them all to have been in the original structure.
For more information here are some websites
http://irishantiquities.bravehost.com//down/ballygraffan/ballygraffandolmen.html
https://www.megalithic.co.uk/article.php?sid=34580
https://de.zxc.wiki/wiki/Portal_Tomb_von_Ballygraffan