Charges are expected to be brought against meat factories in the north-east following a four-month investigation by the Garda National Immigration Bureau.
The charges relate to the illegal employment of up to 100 people, from South Africa and Brazil, in two factories over the past five years. The employees believed they were working here legally.
Insp Brendan Burke of the bureau told The Irish Times a file had been sent to the Director of Public Prosecutions.
"The investigation involved 10 officers over four months. It was a decent-sized investigation. It involved the seizure of a lot of paperwork."
A meat factory in the North may also be involved.
Many of the affected workers have been "let go" by the factories. They are in a "legal limbo", according to Mr Anton McCabe of SIPTU's anti-racism working group.
"Through no fault of their own, they find themselves here illegally. They have no right to work or to welfare payments. Some of them have young families."
It is understood the meat factories were exploiting a loophole in the work permit system known as the intra-corporate transfer. This procedure has been partially suspended by the Department of Enterprise because of its wide abuse.
It is used by companies which have sister companies abroad to transfer workers here for training. Visas are typically granted for periods of six months to a year.
It is understood the companies at the centre of this investigation had been using the procedure fraudulently to recruit workers, usually with no experience in the meat industry, in South Africa and Brazil.
Affected workers interviewed by The Irish Times said they were told to tell Customs at Dublin airport they were trainee butchers coming from named meat companies, to "upskill" in Ireland.
Their passports were stamped at the airport for between 30-day and seven-month stays, although many continued working at the factories for years.
A group of South Africans who spoke to The Irish Times had been at their factory since 1999 and 2000.
They spoke of 12-hour shifts of non-stop physical work, starting at 6 a.m. with three short breaks; of sharing 3-bedroom houses - owned by the factories - with up to 13 others and earning "at most €200 a week".
Apart from canteen food, one man said he lived on bread and cornflakes but would buy himself a burger on Fridays "for a treat".
The money was said by all to be "excellent" by South African standards and they saw working in Ireland as a "great opportunity".
A group of these workers was given a day's notice last week, with the explanation from one of the factory managers that "immigration was on their backs".
Mr McCabe fears others will be similarly dismissed in coming weeks.
He is hoping alternative employers may take them on. The factory owners had "no regard for these people as human beings", he said.
Insp Burke said the immigration bureau would look sympathetically at these particular cases.