Irish authorities are preparing to charge a man for the murder of Denis Donaldson, a British spy within the IRA who was shot dead in 2006.

The news emerged on Wednesday when an Irish police officer told an inquest in Letterkenny, County Donegal, that prosecutors had obtained a warrant to arrest and charge the suspect with murder.

Donaldson operated at the heart of the IRA and Sinn Féin for decades and was a confidant of Gerry Adams until being outed as an MI5 agent in 2005, causing major embarrassment for the republican movement.

Originally from Belfast, after his exposure Donaldson moved to an isolated cottage near Glenties, Co Donegal, in the Republic of Ireland. The 55-year-old was shot dead with a shotgun there in April 2006.

The Real IRA, a dissident group opposed to the peace process, admitted responsibility but there is speculation the mainstream IRA may have committed the murder.

After more than 20 postponements, the inquest heard on Wednesday that Irish authorities were moving to charge the alleged killer. Previously there have been arrests but no charges.

Garda Supt Michael Finan said the the director of public prosecutions had instituted proceedings. “A warrant has recently been obtained from the special criminal court for the arrest of an individual for the purpose of taking him before that court, where he is to be charged with Mr Donaldson’s murder.”

The coroner, Denis McCauley, adjourned the inquest until September 2020 to allow criminal proceedings to progress.

Ciaran Shiels, a solicitor for the Donaldson family, said the suspect was serving a prison sentence outside the country and it would be at least a decade before he could be extradited, according to the news site Donegal Daily.

Before being exposed, Donaldson was a prominent figure in Sinn Féin and ran the party’s administrative team at the Stormont parliament in Belfast.

As a renowned fixer for the republican movement’s leadership, he sidelined awkward or rebellious candidates and replaced them with people who toed the party line.

Donaldson also headed international relations for Sinn Féin and spent time in the Middle East meeting Palestinian factions and Hezbollah in Lebanon, all the while working for British intelligence.

In 2005 he confessed publicly to working for the Royal Ulster Constabulary special branch as well as other branches of the security forces.