There are so many commenters here repeating this mantra "just build higher, just build denser", with seemingly no grip of facts about property in Dublin.

There's no restrictions on height nor density in the Docklands area of Dublin: an area within walking distance of the centre of the city, and with a good regular tram link, that is mostly vacant. Similarly, many areas on major tram routes are not so restricted, such as Sandyford and Cherrywood (these three areas, along with East Point, also happen to be where much of the tech boom is focused). So your denser/higher suggestion is being done and is having no affect—residential rents in these areas are the highest in the city, and much of the units available are left empty as a result.

Meanwhile, in terms of actual supply, even outside of those areas, Dublin has an estimated 40000 vacant residential units, and plenty of vacant commercial units on top of that.

The primary reason for this is that rent prices are not demand-based as much property is financialised—rent isn't a driver for owners, and there are little to no penalties for lack of tenancy, as other cities have.