I cannot vouch for the accuracy of the Railway Network map below.
The map is accurate, but what it doesn't show is that a signficant part of the lines that disappeared between 1920 and 2020 consisted of low-quality, narrow-gauge (3' or 3' 6") single-track lines that largely ran along or alongside existing roads. They were established and designed to compete with the only alternative, which was horse-drawn traffic, and with average speeds of between 20 and 40 km/h (but slower on uphill inclines) they could do that. But, even then, most of them never made a profit. Once lorries and buses came along, their fate was sealed. Even if the money were there, it would have beem impossible to upgrade them to double-track, standard Irish gauge lines; a complete new route would be required.
Letterkenny is mentioned in the thread. In 1920 it was possible to travel from Dublin to Letterkenny by rail. The route would be:
- Dublin to Dundalk on a standard gauge line
- Change at Dundalk or Portadown for Derry, again on a standard gauge line.
- At Derry, change stations from Waterside to Foyle Road. You'll have to do this on foot, or hire a cab.
- Derry to Letterkenny on a narrow-gauge line.
Total travel time is 8.5 to 9 hours, not counting connection times at Dundalk and at Derry, which depend on timetables (and services on the smaller lines were not frequent). The last section, from Derry to Letterkenny, takes
two hours.
There's an alternative which involves alighting at Strabane and taking a different narrow gauge railway to Letterkenny, but the total travel time is longer. Plus the connections might require you to overnight at Strabane, which I wouldn't wish on anybody.
So, even in the good old days the rail service to Letterkenny was dreadful. Donegal had quite a dense railway network of which Letterkenny was actually one of the hubs, but the entire system had a 3' gauge , and average speeds of 15-20 kmh. On certain sections of track an express service could get up to 30 kmh, but these sections were few, and not lengthy. None of this was because Letterkenny was considered unimportant; it was because the terrain in Donegal doesn't lend itself to anything except light, narrow-gauge railways. And of course that hasn't changed since 1920.
Tl;dr: the 1920 rail network was extensive, but much of it was of low quality. Even at the time it wasn't financially viable, so there was no capital available to upgrade it. It couldn't provide a service that anybody nowadays would choose to use and, given the routes along which the narrow-gauge lines in particular ran, even if cost were no object upgrading to standard gauge simply wasn't an option. For many of the lines that were closed, the alternative of investing to upgrade to a service that people would use wasn't available at any price. For others, it might have been possible if funds were unlimited, but it still wouldn't have been a wise use of those funds.
There are, of course, lines that were closed that could have been upgraded, and we might thinks should have been — the branch from Portadown to Derry is often mentioned. But even on the most favourable view the rail network in 2020 was alwasy going to be much smaller than in 1920.