While not disagreeing with the detail of your point, something is fundamentally broken if the two practical alternatives offered within the Irish system to a twentysomething are to invest in property (completely unrealistic) or put those funds away for forty plus years.
Putting a roof over your head in the shape of a PPR is not "investing in property", it's meeting essential accommodation costs via a means other than renting.
Big picture - the situation seems quite absurd if our societal objective had been enabling the ordinary person to forge independence, better themselves and form stable families. As you mention, given how long the situation has prevailed, our real societal objective seems to be something else entirely ... a question I'll leave open.
Well, think this through.
You present the "two practical alternatives" offered by the Irish system as the only savings/investment options open. You then say that "putting a roof over your head in the shape of a PPR is not 'investing in property'". But you yourself have treated it as exactly that in your first paragraph. Is there a bit of a tension there?
Obviously, people buy houses for non-investment reasons, and the transaction delivers benefits which go beyond a financial return on investment. Neverhtheless it is an investment — and, up to now, for most people by far and away the best investment they will ever make , even if we ignore all benefits other than the financial return. If you're above the median wealth in Ireland today, this is amost certainly largely because you bought a house on mortgage.
And then you say "the situation seems quite absurd if our societal objective had been enabling the ordinary person to forge independence, better themselves and form stable families". Actually, up to now the system has worked very well — for the ordinary person to buy a house absolutely has been the key to independence, financial betterment and stable living arrangements, and we have acheived fairly high rates of home ownership.
But the system is not working well
now — for more and more people, home ownership looks unattainable. And those who do attain it are not likely to enjoy the same real growth in property values that their parents and grandparents enjoyed, because it's that rapid growth in property values that has led to the affordibility crisis we have now. The system has run out of road, basically.
What has this got to do with tax-incentivised plans for investing in equities? Well, possibly not much. But possibly this: we urgently need to make housing more affordable, and to keep it that way. I don't have a magic wand that I can wave which will do that, but suppose we do succeed in doing it; what are the implications?
The implications are mostly very good, but one of them is that owning a house will no longer be a long term route to signficant wealth accumulation — the value of your house will be more or less stable, relative to earnings. Which means that, if we want people to accumulate wealth, we need to offer them alternative mechanisms. Something like an ISA might have a role to play in that.
But that's for the future — that case has no traction at all until we have a stable and realistically-priced housing market.