A treetop sky-bridge, visitor centre and cafe have been given the go-ahead as part of contentious plans to develop Dublin's historic Hellfire Club forest lands into a major tourist attraction.
Local groups opposed to the scheme said they were devastated by An Bord Pleanala's decision to approve the project, which aims to triple the number of visitors annually.
They fear the development will damage the landscape and spoil its natural beauty, and that the influx of visitors will disturb local wildlife.
"We're shattered," said Elizabeth Davidson, of Save The Hellfire.
"There is so much opposition to this and we have been fighting it for four years, but it seems we just haven't been heard."
South Dublin County Council is behind the development under an agreement with Coillte, which owns the land.
Hellfire Forest and Massy Woods cover most of the 152 hectare area, which in is the foothills of the Dublin Mountains and commands spectacular views over the city.
Montpelier Hill, at the highest point, holds the remains of a hunting lodge built in the 1700s which became infamous for the antics of wealthy social group known as the Hellfire Club.
The wider area has other architectural features, as well as the remains of two passage tombs, and it is the home of red squirrel, merlin, pine marten and bats.
Central to the €20m development plan is a 1,000sqm visitor centre incorporating a cafe, education centre and exhibition space, as well as a treetop sky-bridge for walkers over the main access road, linking newly enhanced trails in the forests on either side.
Car parking is to be quadrupled to 275 individual spaces plus dedicated coach parking and the overall project is designed to boost visitor numbers from 100,000 a year to 300,000.
An oral planning hearing was held in 2018 and an initial inspectors' report early last year recommended the project be refused permission unless much greater analysis was provided about the possible impact on wildlife.
An Bord Pleanala sought a number of additional assessments from South Dublin County Council.
A fresh inspector's report found the concerns about wildlife had been addressed and recommended the project be approved - apart from the sky-bridge.
The sky-bridge was considered likely to disturb wildlife. Some were concerned that it would have to be enclosed for safety reasons, making the structure too intrusive in appearance and scale.
An Bord Pleanala, however, has granted permission for all elements of the project, subject to conditions including the drawing up of a forest management plan with the National Parks and Wildlife Service and the appointment of an ecologist to oversee the works.
The inspector said the option of "doing nothing" for the area was not acceptable because visitor numbers and traffic were increasing and existing facilities could not cope with them.
Opponents could launch a legal challenge but Ms Davidson said no decision could be made just yet.
"We have to read the inspector's report in detail and take expert advice. At the moment, we're just trying to understand this decision. There are a lot of angry people," she said.
The council has been contacted for a comment.