Turns out that the open source UK maps (http://talkytoaster.info/ukmaps.htm) work fine for off road NI. Download the map file (includes contours), if your GPS uses an SD card - create a folder in it called "GARMIN" save the map file as "GMAPSUPP.IMG", then when you start up your GPS it will load that map file.
The maps are created from open source/wiki generated data, so anyone can add stuff manually to the map here: http://www.openstreetmap.org/ it also allows you to upload your GPS's route file (.gpx), then you (or some other willing party) can 'trace' along the route to make paths/roads/rivers etc.
(http://wiki.openstreetmap.org/w/images/8/86/Osm_linkage.png) (http://wiki.openstreetmap.org/wiki/Main_Page)