To be fair, commerlad, Clare is a major holiday destination with good transport links. From Dublin it's only a few hours on the motorway, and Shannon Airport brings in visitors from across Britain, Europe and North America. From an Irish perspective, it's a little odd seeing all those references to "the mainland" - for many of us, Ireland is the mainland!
That said, I think your post does highlight what is probably the more fundamental issue: scale. The Republic has around 5.5 million people compared with roughly 68 million in the UK. Even if you include Northern Ireland, the all-island market is still much smaller. That means a much smaller pool of enthusiasts, volunteers and skilled people to keep heritage railways running, and fewer local visitors making repeat trips throughout the season. Heritage railways don't survive on overseas tourists alone; they depend on local support year after year, people like yourself making regular day trips to their local railways.
Tourism certainly helps (Clare attracts huge numbers of visitors) but tourism and heritage railways aren't quite the same thing. A tourist might visit the Cliffs of Moher once, whereas a preserved railway needs a loyal base of repeat visitors, members and volunteers, alongside tourists, to justify the ongoing costs of maintaining locomotives, rolling stock and infrastructure.
Ultimately, I think that's the fundamental challenge. Heritage railways need substantial capital, continuous maintenance and, above all, a large and active community of volunteers and repeat visitors. With a much smaller population to draw on, Ireland simply has a smaller base from which to sustain projects like these over the long term.