Revenue officials, gardaí and other agencies have raided fishing vessels in Dublin and Cork as part of an investigation targeting undocumented workers.
The operation remains ongoing and so authorities have declined to comment on the details or whether any arrests have yet been made.
It is understood boats have been targeted in Howth, Co Dublin and Castletownbere, Co Cork.
There was significant Garda activity at Howth station on Wednesday morning but the nature of the operation remains confidential. It is expected to remain ongoing until later in the evening or into Thursday morning.
Ken Fleming of the International Transport Workers Federation (ITWF) said the raids had come following months of work to expose the practices within the industry.
Last February, the State made 500 special permits available for non-EU citizens to work in the Irish fishing industry but to date only 159 had been applied for. Mr Fleming said he estimated there are in the region of 1,000 foreign workers employed in Ireland.
Last year, the Guardian reported the Irish authorities had been aware of the trafficking of migrant workers into its fishing fleet as long ago as 2010 but failed to tackle the problem.
It cited documents obtained under Freedom of Information requests which purported to show trafficking of fishermen from countries outside the EU was raised by officials tasked with investigating illegal employment and the flouting of immigration laws in the sector.