Maurice McCabe's bid to throw light on Garda wrongs is linked with several political crises

But it is the handling of the officer's allegations and how they were acted on, or not, that resulted in the most lasting damage.

Former Tánaiste Frances Fitzgerald is the latest in a long line of casualties.

In the years Sgt McCabe has fought to be vindicated, ultimately securing a public inquiry into his treatment, two Garda commissioners have retired from the force, Alan Shatter resigned as Minister for Justice before being vindicated in the courts, and two senior civil servants have stepped down.

The Disclosures Tribunal is investigating allegations that Garda chiefs orchestrated a smear campaign, including false sex abuse claims, against Sgt McCabe - a scandal which almost brought down the fragile minority Government at the start of the year.

His revelations of malpractice began in 2008 when he was stationed in Mullingar and he raised concerns about penalty points being quashed by senior officers.

He used a Garda database to deepen his trawl only to have those privileges removed.

In the subsequent two years, he documented 42 cases of alleged bad policing in Co Cavan.

An internal inquiry took place but Sgt McCabe was not satisfied and made a complaint to the force's Confidential Recipient.

A second internal inquiry examined the cancellation of penalty points and while it found some rules had been broken it said there was no criminality.

But the issue took a new twist as the Public Accounts Committee began to probe the extent of the loss of revenue. In January 2014 then-Garda commissioner Martin Callinan described the allegations as "disgusting".

Two months later, after the Garda Inspectorate contradicted the Garda's internal inquiry, Leo Varadkar, then the transport minister, described the whistleblower as distinguished. In more recent times he has said his actions were heroic.

In the meantime, Sgt McCabe took his dossier of bad policing to Fianna Fáil leader Micheál Martin, which prompted then-taoiseach Enda Kenny to launch the first of several Government-ordered inquiries

Ultimately Sgt McCabe was vindicated with the vast majority of his concerns upheld.

But controversy around the revelations and how he was treated has come to the boil repeatedly over the last few years.

Leaked transcripts emerged in May last year of hearings behind closed doors in 2015 at the O'Higgins Commission, one of the Government-ordered inquiries into Sgt McCabe's claims of malpractice by gardaí in the Cavan-Monaghan area.

They revealed that lawyers for the then-Garda commissioner were to question Sgt McCabe's integrity and that he may have been motivated by a personal grudge.

An email which only came into the public domain in the last week was sent to Frances Fitzgerald in 2015 outlining how the Garda lawyers were intending to raise a "serious criminal case" at the O'Higgins Commission.

That inquiry went on to praise Sgt McCabe.

One key area of inquiry for the Disclosures Tribunal is the issue of the unfounded allegation of abuse made against Sgt McCabe.

It dated back to 1998 and was made in 2006 by the daughter of a garda who had been disciplined.

The Director of Public Prosecutions found no grounds for prosecution and that there was no evidence the incident ever happened.

It determined the incident was no more than horseplay.

At the start of this year, details of false abuse allegations against Sgt McCabe became public.

Not only had the DPP determined there was no case to answer over the original complaint but health service staff had inadvertently copied the details of a much more serious sex assault case on to a file related to Sgt McCabe's complainant.

The Disclosures Tribunal has examined this and the protected disclosure by the former head of the Garda press office, Superintendent David Taylor, that there was a smear campaign with the allegations of abuse being used to discredit Sgt McCabe among his colleagues, some sections of the media and politicians.

Mr Kenny apologised to Sgt McCabe in February.