Your speculation on why they might be vacant is besides the point and doesn't provide a reason for why our vacancy rate is so out of whack with international markets (which would have old people moving to nursing homes, property needing repairs etc.). Should capital appreciation stop (i.e. the famous slow landing) these glut of properties can and will be added to the market (either rental or property sales). The shear number of them identitied by the CSO is the big worry as it represents many years of building.
Out of whack with international markets for at least one very good reason: High proportion of owner occupiers. So where someone goes into a nursing home (for example), the inertial effect is to churn that property faster rather than to go to the ongoing effort of paying rent; the inertial effect in Ireland with its high proportion of owner occupiers is to leave the property vacant. The reasons for vacancy are relevant; I don't suggest that they make it "right" in some way, and it's certainly not economically efficient.
If the government were to intervene in the property market, perhaps the best ways to do this would be to create incentives to greater efficiency. Annual property taxes for second and subsequent houses unless rented for not less than (say) ten months a year, or registered as a holiday home (which should facilitate infrastructural / schools etc planning in those counties with an especially high proportion of holiday homes.
Many muliple-homeowners these days are what could be termed "accidental investors". Property was bought while they were young and was held on to longer than they needed it because the mantra was "You'd be mad to sell". Their circumstances may have changed since they bought it (nesting with partner, starting a family) however there's not much to worry about if the capital is appreciating year-on-year. Until recently banks were only too happy to help in this process (often suggesting it in fact).
Without capital appreciation the picture changes dramatically though and you can be guaranteed that that's when a lot of these properties will be added to the rental or housing market.
I think a very high proportion of the "accidental investors" have had their properties on the rental market anyway. They generally haven't been so cash rich as to be able to afford two mortgages. And people have been worried about the imminence of a crash since at least 2001 - I was very much afraid I'd bought my house "at the top of the market", and just relieved that it was sufficiently affordable for us to be able to stay there even if we went into negative equity...
"So many times on this thread, bulls posit reasons why there are so many vacant homes. The reasons run the gamut of plausible to ridiculous. At the end of the day, it doesn't matter why these properties are empty, simply that they are. Historically, about 3% of property remains vacant. For some reason, in a country supposedly affected by a severe housing shortage, we have a 13.5% vacancy rate."
... is that supposed to be me? I'm not a bull - I'm more or less neutral. I'm buying and selling PPRs just at the moment, but I wouldn't buy a further rental property. Nor, on the other hand, would I dispose of the one I have. I'm probably close to being a property agnostic.