There are three colour plates of MGWR interest in the Railway Magazine...
A nice loco portrait of 129 "Celtic" in MGWR blue livery, Vol 11, 1902
129 again, this time in Green, hauling a train of brown coaches on the Limited Mail in Vol 31, 1912
no 6 "Kylemore" locomotive portrait, in very dark blue/black, Vol 41, 1917