An Bord Pleanála has approved a multi-million euro road project, aimed at alleviating traffic congestion in Galway City.

The N6 Galway City Ring Road would comprise dual and single lane carriageways, over an 18 kilometre route.

The decision by the planning board follows months of deliberations, after an extensive oral hearing, held over several weeks last year.

The board voted by a majority of four to one to approve the project, with a number of conditions.

They say the design, layout and alignment of the proposed road, along with a range of proposed mitigation measures, will not adversely affect the integrity of a number of European Special Area of Conservation (SAC) sites in the area.

The board was satisfied that an Environmental Impact Assessment report was "up to date".

It also recognised that the demolition or acquisition of 54 dwellings along the proposed route, would have a "profound negative impact for home owners" and would also have a severe impact on the communities involved. But An Bord Pleanála also said there would be long term positive impacts for some communities currently "severed" due to traffic volumes.

It deems the route will have "significant permanent positive impacts" regarding improved journey times, reliability and improved connectivity.

But there will be "significant negative direct and indirect impacts" on archeological and built heritage sites along the route. Some of these will be mitigated by undertaking detailed photographic recording and monitoring during construction. However, one protected structure will be demolished entirely.

An Bord Pleanála says the development must be carried out in full accordance with plans submitted, subject to some conditions. These include a revised design around the Parkmore business park and the abandonment of proposed new permanent stables at Galway Racecourse.

Special infrastructure will have to be provided for barn owls, otters and lesser horseshoe bats along parts of the route. Stipulations are also made about the types of tress that can be planted adjacent to the road.

Supporters of the planned route say it is essential to deal with widespread gridlock in the city. They argue that the future development of the city is largely contingent on traffic alleviation measures being carried out.

But opponents contend that the construction of such a major road would only increase emissions. Many instead favour enhanced investment in sustainable transport options and an increased focus on potential public transport solutions.

A formal application for the ring road was submitted over three years ago.

Since then, Compulsory Acquisition Notices have been issued to around 500 landowners along the favoured route.

The lead agency for the project, Galway County Council, estimates the road would take three years to construct. It would involve a number of tunnels, as well as a viaduct over the river Corrib.

The N6 Ring Road plan follows a failed attempt to construct a by-pass of Galway city more than a decade ago.

The proposed road would cost at least €600 million.