The data from last year isn't published yet, and may well show a pandemic and remote working impact, but the CSO has been reporting a slow but steady urbanisation of the population,
There certainly are some unusued properties in desirable and services aras, but when the need is of the scale of 10's of thousands of additional properties a year, existing dwellings will only scratch the surface. It's a high effort, low gain approach.
Let's face it, the vast majority of these are in private ownership, and you just can't force people to rent them out if they don't want to, Taxation can encourage it, but only to an extent.