don't they both use the same spheroid? or are these the wrong EPSG numbers?
PROJCS["IRENET95 / Irish Transverse Mercator",
GEOGCS["IRENET95",
DATUM["IRENET95",
SPHEROID["GRS 1980",6378137,298.257222101,
AUTHORITY["EPSG","7019"]],
PROJCS["IRENET95 / UTM zone 29N",
GEOGCS["IRENET95",
DATUM["IRENET95",
SPHEROID["GRS 1980",6378137,298.257222101,
AUTHORITY["EPSG","7019"]],
TOWGS84[0,0,0,0,0,0,0],
AUTHORITY["EPSG","6173"]],