An Bord Pleanala has quashed planning permission for a waterside development of luxury houses, apartments and cafe in Dalkey.
Bullock Harbour Preservation Association (BHPA), which took a judicial review, has been told the planning appeals board had consented to an "order of certiorari", resulting in the quashing of its decision.
The community group had argued that an Environmental Impact Assessment should have been carried out before permission was granted.
It also claimed the board did not take into account potential flooding at the site.
The battle to prevent the development has lasted nearly four years.
Among those who opposed the project were singer Christy Moore, who lives in the area, Solidarity-People Before Profit TD Richard Boyd Barrett and broadcaster Pat Kenny.
Local Fianna Fail TD Cormac Devlin confirmed the news on Facebook.
"An Bord Pleanala has conceded the Judicial Review brought by the Bullock Harbour Preservation Association for the proposed development. This effectively quashes the planning permission awarded by the board in 2019," he said.
The board had granted permission on appeal, despite its own inspector recommending that it be refused.
A spokesperson could not be reached for comment.
Bartra Property, which proposed to build two apartments, three houses, a marine leisure centre, a cafe, changing rooms and a square at the harbour, was a notice party to the case.
"Bartra are not a direct party to the proceedings. We note the position of An Bord Pleanala and welcome the fact it has nothing to do with the merits of our proposal," chief executive Mike Flannery said.
BHPA chair Dr Susan McDonnell said: "This is great news for the community of users and residents of Bullock Harbour.
"The proposed development was entirely unsuitable for this site in multiple regards.
"We are so grateful to our supporters from Dalkey and beyond who have made this outcome possible."