Sadly Julian's method of grid refs is mad and bad.

When he uses 160.922193 he means OS Sheet 160 and coords 922 east 193 north. What this doesn't give you is the letters that go with the grid ref. This means it's absolutely no use for converting :-(

Luckily, Holy McGrail added Long/Lat to all the sites on TMA so the one you're looking for can be found here:

http://www.themodernantiquarian.com/site/104

A small warning though. This Lon/Lat can be a 100m or so out. Fine if you're looking for a big thing, not so good if you're looking for a little bit of rock art.

For anyone wanting to convert Irish grid refs you can do it here: http://www.megalithomania.com/conversion/

(Same small warning applies)