741 and 748 were withdrawn due to fire and bomb damage, but were built by BREL in 1974. That being the case, presumably 1974/5 would be the earliest that the DTSOs would have come into service with NIR after 5-6 years in service with BR
Ref Irish Railways Traction & Travel, Peter Jones, 1st Ed. 1987
111 and 112 went to traffic in Feb 1981, 113 in 1984