A few years ago (I will look up my files), I wrote on another website about an alternate history of the Great Northern where the Northern Irish Administration was less hardline on cooperation with the Republic of Ireland, and more 'open' to modernising their part of the system.
Maybe its a sense of a 'tidy mind' on my part, but that rail network "hole" in several counties looks unbalanced.
Northern Ireland is a curious case study. Rail services have 80% of the population of Northern Ireland within a 10 mile radius of a railhead, and its fairly well integrated with other transport modes. But there are significant large towns that are not served, such as Omagh, Strabane and Enniskillen, and no amount of pipe dreams are likely to change that scenario, however much we'd like that. Many of the old formations have been wiped out and used for bypasses.
So what would survive to today, and how:
1. Dundalk to Enniskillen would likely close around 1976. Two trains daily, both locals, connecting at Dundalk. Likely 141/121 hauled with 2-3 coaches.
2. Portadown to Derry would today host the Gatwick Stock and IR Mk3's hauled by 071's for direct Dublin-Derry services until Railcars arrived.
3. Belfast to Enniskillen via Monaghan would likely be an efficient railcar service run jointly between Iarnrod Eireann and Translink
4. Strabane station would be a major hub for buses in Donegal.
But it would have had a hell of a time getting through the troubles, financial constraints and all else.