Actually, that's worth pursuing as a "might have been".
Had, for example, the mass emigration never happened, the railway would have been WAY busier, but there still wouldn't have been more coal, ores or steel. Passenger traffic would have been the big thing. Commuter services would be evident in Waterford, lots more in Cork, Limerick, Galway and Derry. Those out of Belfast and Dublin would be very much greater. We could expect a great deal more double track with locomotives maybe not generally that much bigger, as distances wouldn't be greater, but faster. Would there, for example, have been many classes of "Jeep"-like fast tank engines, suitable for intensive suburban work?
Electrification, in the absence of huge Irish coal reserves, would very definitely have played a part. If we compare places like southern England, where passenger traffic was huge, and see the extensive electrification of the railways there from the 1920s on, we probably have a more accurate picture of what Ireland would have looked like.
Southern electrics on the Achill line or County Donegal?
Yeugh. Yeugh.