> which is why its population is lower now than in the 1800s
I'm not sure that that's correct; modern planning permission as a concept was only really introduced around 1970, at which point the population was 3 million. The population is now 5.2 million. Ireland's depopulation was down to, initially, the famine, followed by almost constant economic crisis from the 19th century to the 1990s. We tend to emigrate very enthusiastically (this is particularly visible in the 1980s, when population actually _dropped_ for much of the decade).
(I do think that clearly with more sensible planning the population would now be somewhat higher, but you're probably talking an extra couple of hundred thousand.)
Interestingly, though, expectations about population did contribute to our current housing and infrastructure crisis. Around 2010, a lot of long-range planning was done on the basis that the population would reach something like 4.8 million by 2040; of course, this turned out to be wildly incorrect.
The estimate seems to have been based on the theory that, after the financial crisis, Ireland would go back to our historical pattern of decades-long recessions and mass emigration, now that that has failed to happen, well, all that planning is dramatically wrong. Ireland now completes more housing units per capita per year than any other OECD country, and it's still not enough. There's a massive housing deficit, the railways are over capacity with no relief until at least 2026, electricity and water infrastructure are under tremendous pressure, and so on. All of this is essentially a consequence of assumptions made 15 years ago that we'd just go back to mass emigration, and thus it was reasonable to have a decade where ~nothing got built.