If the border had never existed would their still be a GSR and GNR? like the big four of britain? as irish railways were heavily influenced by britain and its railways i think this could have been the case. if it was all under one railway (Great Irish Railways?) we would have a very different railway system now, better or worse off who knows. as for if ireland remained part of the empire ireland probably would have been included in the nationalisation and beeching axe would have left us with Dublin cork and belfast mainline and dublin sligo line, some other lines in the south east may have also survived. one can only imagine the locos running. Deltics with some mk3s anyone?