Shell E&P Ireland and its Corrib gas partners have welcomed the EPA's decision to approve an operating licence for the project's gas refinery at Bellanaboy, north Mayo.
The decision represents a "significant milestone" for delivery of natural gas to the Irish market, it has said.
However, Mary Corduff and John Monaghan for the Mayo Shell to Sea campaign described it as a "very sad day for Erris", and said that their group would be examining legal options, including a judicial review of the decision.
An Taisce is also due to meet this week to discuss its stated intention to apply for interim measures from the EU. The organisation's representation at the EPA oral hearing had maintained that licensing a refinery located within a major water catchment was in breach of EU directives.
In a statement, Shell E&P Ireland's deputy managing director, Terry Nolan, said that the Corrib gas partners were "committed to building and operating a world-class facility at Bellanaboy".
"All decisions around the project are taken with due concern for the protection of the environment. We continue to be open to talk to any groups or individuals who may have outstanding concerns around the project," Mr Nolan said.
The Pro-Erris Gas Group also welcomed the decision, and said it called on all parties to "work together and move forward for the benefit of the area and the country in general". It has also condemned last Friday's protest outside the Bellanaboy refinery which resulted in three injuries, and one man being taken to hospital.
Ms Corduff, whose husband Willie won the Goldman Environmental Award earlier this year on behalf of the Rossport five, said that she was "not surprised" by the agency's decision.
"It has never overturned a preliminary ruling yet, and this is another example," she said. "However, there's still a long way to go with this project, as the route of the onshore pipeline has to be determined with community consent - and that is absent".
Ms Corduff said that no one had confidence in the agency. "We tried to notify it about a spillage into a local waterway close to the refinery earlier this year, and no one was available after hours to take our calls. Does the EPA honestly realise who it is dealing with when it is talking about enforcing conditions?"
