The construction of the North South interconnector would see huge devaluation of property and damage to heritage sites, according to the North East Pylon Pressure campaign.
The group attended oral hearings into the construction yesterday at the Nuremore Hotel outside Carrickmacross.
The proposed construction would see 400 pylons built on a route from Meath to Tyrone.
When Eirgrid submitted the plans to An Bord Pleanála last June, more than 900 objections were lodged.
Amy Treacy from the NEPP says that the pylons would be disastrous for the region.
Speaking to Newstalk Breakfast's Kieran Cuddihy, Amy said "there would be several hundred million [euro] - in the region of €600m - devaluation of property along the whole route.
"The impact on our landscape... It is just phenomenal. They're great monsters that can be seen several kilometres away," she explained.
However, she stresses that health impacts on the community remains their number one concern in relation to the project.
Anti-pylon campaigner Padraig O'Reilly said that if the decision ultimately goes against them, there can be a legal challenge to that decision - an option that he says is expensive, but can be done.
"Failing that, there is still the need for Eirgrid to get agreement with landowners - and we don't see that ever happening... It's going to be a long-running saga," he suggested.
