The Supreme Court has upheld a High Court ruling over An Bord Pleanála's granting of permission for the planned North-South Interconnector.
The North East Pylon Pressure Campaign had taken the case following the High Court's decision rejecting their challenge to the permission.
In a published judgment, a four-judge Supreme Court ruled that the High Court's decision was reasonable.
The court had agreed to hear an appeal on issues, including whether An Bord Pleanála was lawfully designated by the State as a "competent authority".
It also agreed to look at whether its functions in that role created a conflict in respect of its role in approving the proposed development.
The Supreme Court found it was a competent authority.
The section of the project in Ireland was granted planning permission by An Bord Pleanála in December 2016 following a planning process which included an eleven week oral hearing.
The decision was subsequently appealed to the Supreme Court.
Speaking outside the court following today's judgement, Eirgrid's spokesperson David Martin said: "It's great news for electricity customers. We are now delighted to be in a position to move forward with the project".
"There are no more legal avenues open to opposition groups in the Republic of Ireland," he said.
"There is a legal case to be resolved in Northern Ireland and we are confident that can be sorted in the next few months and we can get on with building the interconnector then," he added.
Eirgrid's interconnector project comprises a 400kV overhead line circuit linking an existing substation in Woodland, Co Meath, with a planned substation in Turleenan, Co Tyrone.
The project is planned to provide a second high-capacity all-Ireland interconnector alongside the existing 275kV double circuit overhead line between Co Louth and Co Armagh.
Earlier this month a group of landowners in the North won their legal challenge to approval being given for a new cross-border electricity line.
A group formed under the name Safe Electricity A&T (SEAT) issued proceedings to claim the move was unlawful.
Their lawyers contended that a development of such regional significance needed to be signed off by a minister.
The challenge centred on the legal power of civil servants to take decisions without a functioning executive at Stormont.