The two Class Y engines were only short-lived, coming over in 1944, in the context of anything built from the end of WW2 onwards. No.19 did last about as long as the VS class 4-4-0s.
They were actually tested on mainline freight haulage on the NCC lines. What became apparent was a propensity to run hot, which was a factor in their subsequent restriction to shunting work. It was thought that the problem of keeping the axleboxes cool was due to the way the engines had been regauged.
You'll remember that converting to 5'3" was done very simply, by removing the wheel centres from the axles, plus the tyres and crankpins from the centres, and turning them around. This inside-out approach apparently gave the correct gauge without needing anything more other than "light machining" of the wheel hubs.
Absolutely nothing was done to the frames and axleboxes - they remained at the original measurements for 4'8½". This was different to the Woolwich Moguls, which did have their frames spaced at least 6 to 7 inches wider apart for Irish use. Had this also been done to the "Y"s, the stresses which showed up in longer distance running might not have happened, since the axles would have been more correctly supported in relation to the frames.
By contrast, the Jinties/Jockos running in Britain did not often run hot and were used on almost everything within their power capabilities....and sometimes beyond.