You know I hadn't realised this was the case, you're right there is a glaring omission. I've just checked The Megalithic European which has a comprehensive section on the Irish Republic but Julian Cope's map of Ireland in the Introduction leaves the 'six counties' out. They are not covered in The Modern Antiquarian either.

Perhaps the history of sectarian troubles put Julian Cope off foraging around too much - you can't blame him for that as his family often seem to travel with him.

I would very much like to visit myself in the not too distant future - have wanted to see Boa Island for a while now.
http://www.irishmegaliths.org.uk/zCaldragh.htm