There will definately be stuff out there, but nothing very big. When we last spent time on Anglesey we visited two standing stones and a cairn that weren't previously listed.

Your best bet, if you have the time, is to trawl through the NMRW:

http://www.rcahmw.org.uk/data/

Combine it with this site and Multimap and you'll soon get something.

Good luck!

Kammer x