Two men arrested over SF man's murder

The 55-year-old senior Sinn Fein official and close colleague of party president Gerry Adams was killed in April 2006.

He was shot dead at an isolated cottage near Glenties in Co Donegal in April 2006.

He had been living there since his sensational exposure as an MI5 agent the previous year.

Dissident republican group the Real IRA claimed responsibility for the murder in 2008.

The circumstances surrounding Mr Donaldson’s outing as a British agent and subsequent assassination have long been shrouded in mystery.

Gardai said the two men - one aged in his 40s and one in his 70s – were detained in the Donegal area.

They are being held at Letterkenny Garda station.

A long-delayed inquest into the shooting has been adjourned almost 20 times.

Gardai have repeatedly urged the coroner to postpone the probe.

They have cited concerns that it might compromise their criminal investigation.

The delays have been a source of anger for Mr Donaldson’s relatives.

They have launched a legal action against the Irish state as a consequence.

In 2014, the Garda made a mutual assistance request to a police force outside the Republic of Ireland in a bid to gain what it described as potentially “significant” evidential material.

That material was secured in March this year.