I seem to remember CIE holding up the M6 motorway near Galway, when they insisted that the motorway bridge over the single track had to be wide enough for any possible future double track from Dublin to Galway.
That is in fact quite likely, assuming that the Inter-city network survives for long enough for Irish politicians to grasp the fact that fossil fuels are a finite resource. That fact has not been grasped by most of them no more than it has by the bulk of posters on Boards.