The two power grids will be connected by 138km of overhead lines between Moy in County Tyrone and County Meath.
The northern section has still not been granted approval, with a decision not likely until mid-or-late 2017.
It will be taken by Infrastructure Minister Chris Hazzard following a public inquiry.
However, the green-light in the south is seen as a milestone for the project.
EirGrid's proposal to construct almost 300 pylons in Meath, Cavan and Monaghan had been objected to by about 200 landowners on health and environmental grounds.
The Northern Ireland section is being overseen by System Operator for Northern Ireland (SONI).
Robin McCormick of SONI said the inter-connector is needed to address security of supply problems facing Northern Ireland.
"We very much welcome the positive outcome from An Bord Pleanála. It represents significant progress for the project," he said.
The Republic of Ireland's planning body has given approval for the southern section of the £200m cross-border electricity inter-connector project.