There's no restrictions on height nor density in the Docklands area of Dublin
That's a tiny area compared to the demand for housing.
your denser/higher suggestion is being done and is having no affect
Nonsense. Dublin has a very low density compared to any other European city. It's almost rural.
Dublin has an estimated 40000 vacant residential units
...which is clearly not enough to lower rents given the 1347359 inhabitants in the metro area.
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.
True. But investors cannot buy unlimited amounts of property. Increasing supply of cheap and good quality (for a change) apartments would obviously help.