It sounds like the the property wasn't considered suitably habitable for it to be declared as a residence for Local Property Tax purposes. Revenue suggest you consider whether the property has a sound roof and windows, electricity, water, and sanitary services.
If it wasn't habitable, then renovating it to bring it back into use almost certainly requires planning permission and all works to be compliant with the current building regs. Depending on your local authority and their feelings on the suitability of the development, they may seek to have your work undone. See
here for an example of an authority addressing an unauthorised development where a couple mistakenly thought a 30k renovation on a 400k property was exempt.
As MelF says, lots of people have installed garden rooms and have let them via AirBnB and the likes, but that is illegal even if not heavily enforced.