Reading Edwin McMillian's book 'Dark Days & Brighter Days for NIR' I came across further confirmation on Hunslet's information above.
"In Dec 1980, six new 50T capacity, vacuum-braked bogie rail carrying wagons were delivered to NIR. Built by CIE at their Inchicore in Dublin Works, each wagon was fitted by two petrol driven crane units enabling a total load of 50 rails to be loaded or unloaded by two men within about a half an hour. The new wagons were numbered C476 to C486, and replaced NIR's existing 40T bogie timber trucks, which dated from 1911'.
I think Flying Scotsman may have done some models of these a while back.