The High Court has rejected an appeal against a €280m power line between the Republic and Northern Ireland.
The North-South Electricity Interconnector will see EirGrid build 400 pylons between Meath and Tyrone.
Supporters say it is vital for Ireland's energy market, while those opposed to it say it will have negative environmental and health impacts.
The appeal was taken by the North East Pylon Pressure campaign against An Bord Pleanála's decision to give it the green light.
Spokesperson Pauric O'Reilly says he is disappointed but not surprised.
He said: "We understand the need for it, we support the need for it, but from the very outset - which is 10 years ago now - we put a lot of money and effort into looking into an underground option along public roads.
"This would obviously eliminate all the issues that people have in relation to health concerns."
EirGrid welcomed the decision, their director of grid development and interconnection, John Fitzgerald, said: "The North South Interconnector is undoubtedly the most important infrastructure scheme on the island today and will help deliver very real benefits to domestic and commercial customers.
"We look forward to moving on to the next stage of the project, in particular engaging with landowners in Meath, Cavan and Monaghan who are directly affected by it."
When Eirgrid submitted the plans to An Bord Pleanála, more than 900 objections were lodged.
Landowners and residents raised numerous concerns about the project during a 12-week oral hearing into the development in 2016.