A very obvious "might-have-been"; when the Ulster Railway was narrowed, and the Dublin & Drogheda widened, what if the British government had said "we're going 4ft 8 1/2, so Ireland must also"?
The Ulster's 6 ft 2 was going to go anyway, so everything here bar the odd narrow gauge line would have ended up the same standard European gauge.
Cue an Ireland like a giant Isle of Wight with all manner of second hand British pre-grouping stock all over the place. Stanier tanks and Black 5's all over the NCC.... Inchicore never building anything but maintaining into the sixties fascinating but motley collection of old GWR, LNWR and NER locomotives..
So what would have we had instead of blue 4.4.0s on the main line? What would we have today on the main line system (probably lots of Mk 3's, for starters!)