The investigation relates to certain parts of Munster and Leinster
The Competition and Consumer Protection Commission (CCPC) is investigating potential bid rigging in the procurement of publicly-funded transport services.
The probe relates to certain parts of Munster and Leinster.
The CCPC says it opened an investigation in 2016, following an examination of a complaint.
As part of the investigation, the CCPC applied to local District Courts for search warrants to gather evidence.
The agency undertook 20 searches in Tipperary, Waterford, Limerick and Kilkenny last July.
It says authorised officers from the CCPC and several members of An Garda Síochána - including a member of the the Garda National Economic Crime Bureau - carried out the searches.
In a statement, the CCPC says: "This investigation is ongoing and the CCPC encourages any individual who has evidence to come forward and contact them.
"In addition the Cartel Immunity Programme operated by the CCPC, in conjunction with Director of Public Prosecutions, means that a member of a cartel may avoid prosecution if they are the first member to come forward, reveal their involvement in illegal cartel activity and fully co-operate with the investigation."
Isolde Goggin, chairperson of the Competition and Consumer Protections Commission, added: "Markets work best where businesses vigorously and independently compete against each other for customers.
"Cartels, where businesses offering the same products or services collectively agree not to compete with one another or to fix prices, cause very significant harm to competition and consumers.
"Bid rigging in public procurement can be particularly harmful as it can artificially increase prices and ultimately costs taxpayers more and can reduce the quality of services the State provides."
Calls to the Department of Transport, Tourism and Sport went unanswered at time of publication.