The move was confirmed in a letter to the Donaldson family, in which Fiosrú – which succeeded the Garda Síochána Ombudsman Commission (GSOC) – said it believed the Garda had not complied with a request for information deemed necessary to the inquiry.

As a result, it stated that it had initiated proceedings in the Dublin Circuit Civil Court, seeking an order compelling disclosure. The case is at an early stage.

The Donaldson family’s solicitor, Enda McGarrity, said the Fiosrú investigation had been ongoing for three years. “We now understand that Fiosrú are commencing legal proceedings against An Garda Síochána arising out of a failure to comply with aspects of the investigation,” he said.

“That’s a matter of significant concern to the family because the body which is charged with investigating the murder of Denis Donaldson are now not only being investigated, but failing to comply with investigation and that gives rise to the family’s wider concern that the State are currently not delivering an effective investigation.”

Taoiseach Micheál Martin said on Friday that he would be willing to meet with the Donaldson family to discuss their concerns.

Mr Donaldson, a prominent Sinn Féin figure at Stormont, was exposed in late 2005 as a long-term agent for British intelligence. The revelation came after criminal proceedings related to an alleged IRA spy ring collapsed. He was shot dead in April 2006 at a remote cottage near Glenties, Co Donegal, weeks after being visited and photographed by a journalist.

Three years ago, his daughter Jane submitted a formal complaint to GSOC, raising concerns about Garda actions and alleging a failure to act on warnings from the PSNI. She said the PSNI had informed Gardaí of an increased threat to her father’s life following media contact, but that no adequate measures were taken to protect him.

While dissident republicans later claimed responsibility for the killing, the family maintains an “open mind” about who was ultimately behind it.

Former Sinn Féin president Gerry Adams successfully sued the BBC over a documentary which claimed he had sanctioned the killing. Mr Adams had strongly denied the allegation.

An inquest into Mr Donaldson’s death has been postponed 27 times due to the ongoing murder investigation.

Separately, the family is pursuing a civil case against the PSNI and others, following a report from the Police Ombudsman for Northern Ireland which found that the force had failed to carry out a formal updated risk assessment after the media made contact with Mr Donaldson.