The 11 men and one woman were held over public order offences and were taken to Bellmullet Garda Station.
Gardaí say a file is being prepared for the Director of Public Prosecutions.
The 12, who are members of a newly formed community group, were taking part in a protest at Glengad Beach where the pipeline is due to brought ashore.
The group is opposing the location of the €300m gas refinery 9km inland at Bellanboy.
The group says it wants the gas refinery relocated to an area on the north coast of Mayo where they say will present less of a health and safety hazard to the local community and the environment.
