Up to 15 men attacked the Shell Glengad landfall site in Co Mayo overnight.
Some reports said the men were masked and armed with iron bars and chains.
It is reported that they used bolt cutters to access the fencing compound.
Gardaí said the men used a company digger on the site and caused considerable damage to the compound.
There are reports that there were around 30 protestors surrounding the outside of the complex.
The area has been sealed off for technical examination.
Yesterday, a Shell to Sea protestor placed himself under a truck at the entrance to the site where security fencing was being erected.
