The Government is facing calls to set up a public inquiry into the shooting dead of a Garda officer and the injuring of a young mother by her abusive partner.
Adrian Crevan Mackin, who was charged with being a dissident republican, was on bail when he shot dead Garda Tony Golden in October 2015 in Omeath, Co Louth.
He shot his former girlfriend and mother-of-two Siobhan Phillips several times in the same incident as the officer brought her to collect belongings from the house she shared with Mackin.
The gunman killed himself after the shooting.
The Garda Ombudsman confirmed it launched an inquiry six months ago into the incident amid concerns that Mackin was a Garda informer.
The investigation is examining how Mackin was handled by detectives after being arrested on suspicion of firearms offences but charged with the more minor offence of IRA membership.
An RTE Investigates programme reported that Mackin was arrested after the FBI notified gardai of his alleged involvement in attempts to buy component pieces for firearms.
The programme said he admitted firearms offences in interview with detectives but was only charged with IRA membership, even though he denied being a member of a terror group.
It was also reported that Mackin was kicked off the republican wing in Portlaoise.
Lawyers for Ms Phillips and her parents Sean and Norma, Belfast-based Madden & Finucane solicitors, said: "This is a significant development in relation to the family's pursuit for information and accountability ... and the serious threat to the family beforehand.
"These revelations raise issues of significant public importance and require an investigation at the highest level."
Solicitor Katie McAllister said the Phillips family instructed lawyers to take High Court action against the Irish state in Dublin.
Gardai cannot run informers "off the books" and all official informers must be registered on the list of Covert Human Intelligence Sources (Chis).
Mackin had an illegally held Glock handgun when Garda Golden, a father of three, went to the house on the Mullach Alainn estate in Omeath with Ms Phillips and her father Sean.
Mr Phillips heard the gunshots and called emergency services.
The Garda Ombudsman said its investigation was in the "public interest" and was launched after it got information and complaints from several sources.
"Inquiries are at an early stage, following communications with the Garda Siochana, with the affected families and their representatives and with other interested parties," the watchdog said.
"Gsoc is very aware of the tragic context of this investigation and sensitivity to this will be a priority."
The Garda Ombudsman will examine if gardai "acted appropriately" knowing that Mackin had access to guns and whether they mitigated the " serious risk" this potentially posed to Ms Phillips, the public and gardai.
It also said it would look into the extent and nature of interaction between gardai and Mackin, if Ms Phillips and her family were treated properly by gardai and their complaints acted upon appropriately or if there was a delay in the Garda review of the incident.
"Full and timely cooperation of the Garda Siochana and other organisations will be critical to the effectiveness and progress of this important investigation. The Garda Siochana Ombudsman Commission will use all its legal powers to try to ensure this cooperation," the watchdog said.
Fianna Fail's justice spokesman Jim O'Callaghan said: "If Mackin was a Garda informant, as was suggested by the programme, and was prosecuted with an offence that would facilitate his informing, rather than the serious offences to which he admitted, then this needs to be explained by the relevant authorities.
"Garda operational matters are not the concern of politicians but if decisions are made by state authorities that expose the community to danger, then those decisions must be explained and subject to scrutiny."
Sinn Fein president Gerry Adams said: "The evidence would indicate that Mackin was working as a Garda agent at the time of his arrest in January 2015 or subsequent to it.
"As a result the Gardai did not charge him with firearms and explosives offences which he admitted to during his questioning in Dundalk Garda station.
"Subsequently his bail was significantly reduced and he was released from Portlaoise Prison.
"The Garda also failed to thoroughly investigate the information in their possession about other weapons which Mackin had access to."
Mr Adams said he has written to Taoiseach Enda Kenny four times and the Tanaiste eight times about the case and said their responses have been unsatisfactory.
"All of the families affected by this need to have truth about the circumstances of Mackin's arrest, questioning, charging and relationship with An Garda Siochana," he said.
The Policing Authority welcomed the Ombudsman's inquiry, adding: "The authority places ongoing considerable emphasis on public confidence in the rule of law.
"Investigations in the public interest such as this are an important element in ensuring continued public confidence in our policing service."