Hi Willblunden
Although not Irish prototype, 5ft3in gauge was also used by the Victorian Railways and South Australian Railways in Australia.
There's now quite a collection of VR items on broad gauge. Most will be under 'VR' and 'BG' on the DLS, if they have BG versions (note, some may not list the gauge - these may be SG or BG).
Some VR items may fit with the Irish BG system. Mainly 4 wheel wagons with 'screwlink' or '3 link' couplers, however my 'VR WT Wagon' on the DLS, under the username 'S301', has 'transition' couplers to convert between auto coupled and'hook and chain' coupled stock.
Not sure specifically what else is available in 5ft3in, and haven't seen any Irish prototype models on this gauge.