Up to 90 people have been cutting turf throughout the night on Clonmoylan Bog in Co Galway in defiance of an EU directive and Irish law.
They brought turf-cutting machinery onto the bog yesterday evening and blocked off all access roads with more machinery.
A group of 30 turf-cutters remain this morning and they are currently cutting turf on the Barroughter Bog.
Gardaí are at the scene.
The bogs are protected under an EU habitats directive and restrictions on turf-cutting are in place.
53 bogs in Ireland are protected under the EU directive which has been transposed into Irish law.
Campaigners opposing the bans say they restrict their traditional right to cut turf.
Arts, Heritage and Gaeltacht minister Jimmy Deenihan however has warned that Ireland faces large fines if the EU directive intended to protect unique habitats is not complied with.