Ordnance Survey of Great Britain, http://www.gps.gov.uk
Recommended by OSi and OSNI for all horizontal CTs in the Republic and Northern Ireland.
Derived at 183 points, accuracy 0.4m (95%). Use iteration for reverse transformation ETRS89 to TM75. Emulated by NTv2 method (code 6946) and used as approximate CT to WGS 84 (code 1042).