An illegal dump and diesel-laundering plant has been uncovered close to the border.
Gardai made the discovery as they raided a premises near Castleblaney, Co Monaghan.
Customs officers and members of the Environmental Protection Agency from Monaghan County Council were also involved in the search operation at Reban, Shantonagh.
The area was sealed off and a forensic technical examination of the scene was under way.
A garda spokesman said no arrests have been made.
"The operation is ongoing," he added.