Honour compels me to mention the ex SLNC Z class tanks, 26 and 27, both of which passed to NIR stock - the only non-WT operational steam locos on NIR books - the GNR locos (UGs 48 and 49) stored at Grosvenor Road may have technically been in NIR stock, but they never ran as such AFAIK.
The two ‘Loughs’ were, incidentally, the last named locos to run on an Irish railway under company ownership and operation.
This photo of ‘Lough Erne’, linked from Flickr, was taken under NIR ownership in 1969. 27 lasted almost till the very end of steam on NIR.