Biffo, Just saw your post but if you haven't got it sorted yet, here goes. The "J" in the coordinate IJ 60279 ITM 20651 replaces the first number of the Easting and Northing ref. The J would be replaced by a 3 for E & N in the 12-figure grid ref you're probably more used to seeing.
In this case J 60279 20651 should be 360279 320651.
You'll need a key or graphic which shows how this works for each zone of Ireland but I haven't yet worked out how to post this.
PM me with your email address & I'll send you the graphic, otherwise the Letter designations are as follows (Letter, E, N):
A:0,4, B:1,4, C:2,4, D:3,4, E:4,4, F:0,3, G:1,3, H:2,3, J:3,3, K:4,3, L:0,2, M:1,2, N:2,2, O:3,2, P:4,2, Q:0,1, R:1,1, S:2,1, T:3,1, U:4,1, V:0,0, W:1,0, X:2,0, Y:3,0, Z:4,0.
In other words, if you had a 12-figure grid ref such as 144444, 144444 ( for somewhere near Limerick) it would look like IR 44444 ITM 44444 on your Etrex.
Hope this helps,