The Environmental Protection Agency has begun hearing objections to the granting of a licence for the proposed €900 million Corrib gas refinery and combustion plant at Bellanaboy, Co Mayo.
Shell E&P Ireland Limited has already been granted provisional approval for the integrated pollution prevention control licence (IPCC).
Shell E&P Ireland, An Taisce, the Erris Inshore Fishermen's Association, local priest Fr Michael Nallen and individual members of the Shell to Sea campaign are among 12 appellants to the preliminary ruling, which was published in January.
EPA chairman Frank Clinton and vice-chairman Andrew Fanning will form the two-man panel adjudicating the process. Witnesses can be cross-examined by Shell and other objectors when they give evidence.
Following the hearing, which is expected to last for two or three weeks, a report and recommendation will be submitted to the EPA board before a final decision on whether or not to grant a licence will be made. It may be several months before this decision is made.
The EPA's preliminary approval states that it is "satisfied" that emissions, "when operated in accordance with the conditions of the proposed licence, will not adversely affect human health or the environment and will meet all relevant national and EU standards".
To date, the EPA has not overturned an interim decision of this nature.
If approved, the proposed decision provides for the processing of 9.9 million cubic metres of natural gas per day. The gas will be transferred by pipeline into the Bord Gais Eireann (BGE) distribution network.