Kieran-there are a couple of picture references to the siding at the Courtauld's factory.In Rails around Belfast,there is a photo of one of the two 0-4-0 saddle tank engines with one of the open wagons behind it.These engines were named Wilfred and Patricia and were eventually scrapped after the closure of the factory.In the Ulster Transport Authority in colour book,Jeep No4 can be seen at the head of a long coal train destined for Courtauld's.Most of the wagons are the ones the DH was shunting,these were all refurbished for the contract.There are a couple of grey opens at the front of the train,probably ex BCDR wagon.
With regards freight on NIR,as the UTA had decided to abandon all freight within Northern Ireland in favour of road transport,in fact as they tried to abandon the railway full stop,there was not much freight left when NIR came into existence.As John has pointed out,the Courtauld wagons were used for ballast and sometimes sleepers along with some of the older grey open wagons.I remember the DH doing this work on the GNR section,I have a photo somewhere of it with 3 grey wagons that had their sides cut down to 2 planks.Some of the spoil wagons were also used for ballast.These were converted by welding the side entry doors and changing them to a normal hopper operation.I have no idea how many of these wagons NIR converted.