The Competition and Consumer Protection Commission (CCPC) says it occurred over a 21-month period between 2015 and 2016.
That took the form of public announcements of future private motor insurance premium rises, as well as other contacts between competitors, all of which reduced levels of competition between the parties.
In a statement the CCPC says the findings are provisional and that no conclusion should be drawn at this stage that there has been a breach of competition law.
Those named in the report - AIG Europe SA, Allianz PLC, AXA Insurance DAC, Aviva Insurance Limited, FBD Insurance PLC, Brokers Ireland (formerly the Irish Brokers Association) and AA Ireland Limited - can now respond to the findings and the investigation is ongoing.
In their statement, the CCPC said: "Price signalling and other types of anti-competitive cooperation between competitors ultimately lead to increased prices for consumers.
"These practices are particularly harmful to consumers when they occur in sectors like private motor insurance where motorists are required by law to take out cover and cannot avoid price increases.
"In the course of the investigation, the CCPC gathered a substantial amount of electronic material from relevant parties, as well as extensive oral testimony and documentary evidence through witness summons hearings and meetings.
"The CCPC has now issued preliminary findings to these organisations setting out its position that it has reasonable grounds to suspect that a breach of the law has occurred."
