Opposition is building from environmentalist, local residents and politicians to plans by Transport Infrastructure Ireland (TII) to upgrade the N11/M11.
Green Party leader Eamon Ryan said if his party was in government before the road was built it would shelve the project.
TII last week briefed local people on plans to upgrade a 22km stretch of the N11/M11 from south Dublin to Ashford in Co Wicklow.
The upgrade includes about 2.5km through the environmentally sensitive Glen of the Downs which was the site of a three-year “Save the Trees” campaign from 1997 to 2000, before the road was widened from single carriageway to dual carriageway.
TII has now proposed to add a third lane to the N11/M11 from St Columcille’s Hospital in Loughlinstown in south Dublin to north of the glen. It has offered three alternative route options to address the problem of traffic congestion on the route through the glen.
Two of these on the eastern side intersect on Delgany Golf Club while the third, a western bypass of the glen, crosses very high ground on Downs Hill, which is home to a prominent ring fort.
A fourth option was to widen the existing road through the glen, taking a slice of the special area of conservation (SAC) for the second time in 20 years.
But this week environmental activists and some members of the original Save the Trees campaign – including Mr Ryan – have been regrouping on social media, with a number pledging to oppose any further land grab for road widening in the glen.
On Wednesday night, local residents held a meeting at which they formed the Glen of the Downs Group. Speaking afterwards, committee member Ross O’Connor said his farm would be split up if either of two options on the eastern side went ahead, while his and his neighbours’ homes and a local garden centre would be demolished.
The meeting was also attended by a number of people who said they would lose their homes, or else have their farmland divided, should the western route be chosen.
Mr O’Connor said the group was not enthusiastic about continually widening the glen. Much of the conversation had focused on public transport alternatives, including contraflows and dedicated bus lanes, he said.
Local councillors Jennifer Whitmore (Social Democrats), Tom Fortune (Independent) and Derek Mitchell (Fine Gael) said they were very surprised by the options being considered by the road designers. They said they believed TII would ultimately settle on widening the Glen of the Downs road.
Ms Whitmore suggested a dedicated bus from Co Wicklow to link up with the Luas at Carrickmines, a notion supported by Mr Mitchell, who has proposed an express bus corridor. Mr Fortune said he was in favour of public transport alternatives but added that the project timeframe was a decade away and “may never happen”.
Mr Ryan told The Irish Times he had been one of the original protesters and had been up a tree with other activists during the campaign.
He said: “Twenty years ago we argued for public transport solutions rather than widening the N11 at the Glen of the Downs. The same argument still applies today. Widening all the motorways approaching a gridlocked Dublin is pure madness. We have to change our ways.”
Maps of the scheme and route options are available on n11m11.ie.