Dublin City Council has secured a Compulsory Purchase Order on the square in south Dublin under threat of being turned into a car park.
The council sought the right to purchase Dartmouth Square after its owner closed it to the public earlier this year.
Businessman Noel O'Gara said he wanted to turn the land into a car park.
However, in its decision on the matter today, An Bord Pleanala says maintaining the area as a public park would satisfy a community need and be in accordance with the Dublin City Development Plan.
It also says the park is an element of an architectural set-piece comprising the surrounding Victorian houses, which are already legally protected.