Government labour inspectors are investigating more than 300 cases of possible exploitation and mistreatment of employees, the majority of which involve immigrant workers.
Of the cases examined by inspectors so far this year, the majority are in the hotel (117), catering (85), retail grocery (71), security (44) and agriculture (17) sectors, according to figures supplied to The Irish Times.
Investigations into employment law violations include: employers not paying workers pre-arranged wage rates; workers being paid below the minimum wage and being subject to excessive working hours; illegal deductions; non-payment of overtime or holiday pay.
Officials say that while records of workers' nationalities are not kept, the sectors being examined are those where there is a higher concentration of immigrant or migrant workers.
Almost 4,000 inspections of premises have taken place this year, according to latest figures. Twelve employers have been prosecuted for breaches of employment law.
The figures are down on last year when more than 7,000 inspections took place, resulting in 20 successful prosecutions.
Officials at the Labour Inspectorate, based at the Department of Enterprise, Trade and Employment, say the numbers have fallen due to work commitments related to Ireland's presidency of the European Union.
SIPTU's regional secretary for the midlands and the south-east, Mr Mike Jennings, criticised the inspections regime.
He said the limited number of inspections, along with the low levels of prosecutions, meant many employers would continue to exploit foreign workers.
Mr Jennings said he was aware of a number of cases in which migrant workers were not being paid the same wages as their Irish counterparts.
In one case, he said, a large number of Eastern European workers at a factory were not given pay increases due to them under the national pay deal, unlike Irish workers employed in the same jobs.
He said large numbers of workers from EU accession countries were beginning to join unions such as SIPTU as they became more aware of their rights and entitlements.
Representative groups for migrant workers, such as the Migrant Rights Centre, however, say many workers are unable to join unions and are even more vulnerable to mistreatment.
In a recent study it found that many domestic workers in middle-class homes were underpaid, overworked and not given holidays there were entitled to. There have been calls for the work permit system, described by some immigrant support groups as a form of "bonded labour", to be overhauled.
A spokeswoman for the Minister for Enterprise, Trade and Employment, Mr Martin, said a new Employment Permits Bill is being drafted at present, which may addressed some support groups' concerns.