I'm working on the process for exporting observation data from iSpot to recording schemes. iSpot locations are stored as lat/long, and I am converting these to easting/northing coordinates using OS's Grid Inquest software, and then taking the resulting coordinates and converting them to standard six-figure grid references.

However, for Ireland the Grid Inquest software offers a choice of using the Irish Grid (IG) or the Irish Transverse Mercator (ITM) systems, and the coordinates that result from each of these systems are slightly different; or rather, the 100km square reference numbers are very different (but easy enough to work out how each system maps to the 100km letter references), while the remaining coordinate numbers are slightly different. For example, the lat/long:
52.60002401/-7.811107635

produces IG coordinates of:
212848/149833 (which translates as S 128 498)

but the ITM coordinates are:
612796/649878 (which translates as S 127 498)

The differences in resulting grid ref are small, but clearly there are differences.

I haven't been able to find any information on when the change from IG to ITM happened/will be happening (is it still work in progress?). Can anyone enlighten me on the implications, and on which of the systems I should be using in order to provide the best grid refs for the Irish records that I'm supplying to those recording schemes that cover both Britain and Ireland? Which system underlies the mapping in Recorder/MapMate/NBN?

Thanks,
Martin