There are five state inquiries and one tribunal under way, looking into issues including ticket- touting, the IBRC, a fatal shooting, abuse allegations at a foster home, and a Garda smear campaign.
A further eight inquiries have been promised but have yet to start, while another has already ended this year.
The Government has also appointed a judge to assess new evidence in order to recommend whether a Commission of Investigation should be established into the Stardust Tragedy.
The Department of Justice has yet to set up five inquiries recommended by the Independent Review Mechanism into complaints against the gardaí.
While at least €21.85m will have been spent on just six of these investigations by the end of the year, it is likely millions more will be spent on the Charleton Tribunal.
The Disclosures Tribunal, led by Supreme Court judge Peter Charleton, was set up in February and has been tasked with investigating an alleged smear campaign against whistleblower Sergeant Maurice McCabe.
Past tribunals have posed a significant financial burden on taxpayers — the Planning Tribunal cost €159m while the Moriarty Tribunal cost around €42m.
One of the most cost- effective probes is the non-statutory inquiry into the Olympics ticketing scandal. It is due to complete its work by the end of this month at an estimated cost of €300,000.
At the other end of the scale, over €10m is expected to have been spent on the IBRC Commission, which includes the sale of Siteserv to Denis O’Brien, when it winds up at the end of this year.
A Department of the Taoiseach spokesperson said: “The IBRC Commission indicated in its interim report that significant third-party costs have been incurred which are likely to arise for payment in due course.”
In 2014 the MacLochlainn Commission was set up to probe the shooting of Ronan MacLochlainn by a member of the gardaí during a 1998 attempted robbery of a security van in Co Wicklow. The investigation, led by Mary Rose Gearty SC, has so far cost €1.153m. The Mother and Baby Homes Commission of Investigation, established in 2015, has cost €4.4m.
The Grace Inquiry, looking into allegations of sexual abuse of children with severe intellectual disabilities at a foster home in the South East, will cost €5m over two phases, €2.5m of which will be spent this year.
The Fennelly Commission looked into the retirement of Martin Callinan and recording of phone calls in Garda stations and cost €3.5m when it concluded last month.
Departments could not provide estimated spends for probes into the Stardust tragedy; review of the gardaí; and the Project Eagle sale.