The High Court has rejected a challenge taken against an An Bord Pleanála decision allowing a €280 million power line to be built connecting the Republic and Northern Ireland.
Mr Justice Max Barrett stated there was "no lawful reason".
The North East Pylon Pressure Campaign took the case against An Bord Pleanála's decision to allow EirGrid to build an electricity line and 300 pylons between Meath and Monaghan.
Backers of the project claim joining grids on both sides of the border is vital for the island of Ireland's energy market.
Those opposed to it claim it will have negative environmental and health impacts.
The North-South line is part of EirGrid's national project to secure supplies and modernise the transmission system in projects that could cost anything from €2.7 billion to €3.9 billion, depending on the choices made on the voltage lines.
An Bord Pleanála approved the plan last year, citing it as the most cost-effective solution.
EirGrid said the interconnector will improve the security of electricity supply across the island of Ireland, improve efficiency, reduce costs and save customers money.
There will be 409 pylons on this line in total, including the Northern Ireland section.
Up to 250 farmers and landowners between Dunboyne and Carrickmacross back the pressure group.
Padraig O'Reilly of the North East Pylon Pressure Campaign said he is disappointed by the decision but not surprised.
The campaign group is not opposed to the interconnector, he said, but is opposed to it being overground.
Speaking on RTÉ's News At One, he said: "We have to read the judgment and see what the arguments and the statements of the judge were, which is common sense.
"But, assuming that many of our points are still valid, we will look at the possibility of an appeal to the Court of Appeal or, indeed, it may go directly to the Supreme Court.
"They are the options for this specific process in terms of what happens next."
EirGrid issued a statement saying it welcomed today's court decision.
It said the North-South interconnector is a critical electricity grid development project linking Ireland and Northern Ireland.
EirGrid's 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," he added.