Often the issue is where exactly those vacant dwellings are. Typically, booming metro areas will have a low vacancy rate, while the vacancy rate will be high in rural areas or declining cities, where economic prospects are more dubious.
Demand is local. If someone needs to live in Dublin for their work, it doesn't help them much if there's cheap housing a few hours away.
The only two studies I'm aware of that look at this (admittedly from ~10 years ago and for England and the city of Melbourne in Australia) suggest that the proportion of vacant properties is highest where prices and capital gains are greatest. Not intuitive, but that's what they show.
But yes, it would be helpful to see the breakdown of that census data.
Edit: There is actually a breakdown by county if you scroll down the page. Dublin and Cork appear to be at 6 and 7% respectively.
Quite, indeed in London the pressure group "action on empty homes" say it's 2.2% - and that includes second homes (people with a flat in London and a house elsewhere, airbnbs, etc)
That's a tiny number. Even if that number was zero there would still be a massive shortage. Most claims of "homelessness" ignore overcrowding, they're looking at people living on the street, or at most sofa-surfing, they don't look at overcrowded houses, at 30 year old adults with above-average wages who have to share family homes with half a dozen strangers in HMOs.
I don't know about Dublin but I would be surprised if it's not a similar demographic. A lot of cities have a fair amount of airbnbs because they work better for modern travellers than hotels
The biggest places outside the City of London (the square mile in the very middle, it's a special case due to its size) are holiday hotspots like Cornwall, South Devon, the Lake District, where holiday homes, either private or rented out ("airbnbs") make up nearly 10% of the stock.
They also (like action-on-empty-homes) claim airbnbs as "empty", which is a political view. There are about 200k visitors in a given peak summer week to Cornwall [0] and 13k "second homes" [1]. Assuming they are all holiday lets, and lets go for a typical 4 person family, that's 50k visitors. Slash those numbers and that's a hell of a lot of tourists not spending money, and a hell of a lot of jobs not being funded.
On the other hand cornwall could have its total housing stock increased by 5.6% and all the problems would apparently be solved - as there would then be the same number in primary residential use as exist right now.
Demand is local. If someone needs to live in Dublin for their work, it doesn't help them much if there's cheap housing a few hours away.