The Corrib Natural Gas Field, operated by Shell off the northwest Irish coast, has drawn environmental protests against the laying of a pipeline between the gas field and the mainland and local people fear pollution.
Police said one environmental protester, removed by security guards on Thursday from the compound, was taken to hospital.
The Shell to Sea protest group said an activist was beaten by masked men after he stayed under a truck for hours in a bid to halt work at the Shell site in County Mayo, western Ireland.
A spokeswoman for the Irish police could not say if there were links between the intruders and environmental protests.
Shell said it could not comment on the incident late on Wednesday night as an Irish police inquiry was ongoing.
“There’s been a history of protests with this project but …. this is a new development, we’ve never seen anything to this extent,” Chief Superintendent Tony McNamara told RTE radio.
Police said the intruders wearing balaclavas used the digger they found on the site to damage fences and other objects before making their escape, adding their motive was not clear.
“One member of the security staff received an injury to his arm and received medical attention,” police said in a statement. It was not clear how the guard was injured. McNamara said security staff “fled for their lives”.
The Corrib field, estimated to contain 1 trillion cubic feet of gas, has been beset by protests and delays since its discovery in 1996. Local people want Shell to process the gas on an offshore shallow water platform as they fear that onshore processing would bring the pipeline too close to their homes and pollute their water supply.
In September the army made safe a small explosive device outside Shell’s Dublin offices.