THE outing of Mr Donaldson as an MI5 and PSNI agent almost 20 years ago rocked the republican movement to the core.
From the Short Strand area of east Belfast he is believed to have joined the IRA in the mid 1960s and is said to have taken part in the so-called Battle of St Matthew's in defence of the Catholic church after it was attacked by loyalists in 1970.
Viewed by many as a key moment in the emergence of the Provisional IRA, the night-long gun battle between republicans and loyalists in east Belfast resulted in three dead and several others injured.
A former republican prisoner he was close to IRA hunger striker Bobby Sands and is often seen pictured with him in an iconic photograph taken in 1975.
As the peace process evolved he played a central role as part of Sinn Féin's backroom team and was known to be close to the party's former president Gerry Adams.
Sources say one of the his roles was to act as a 'link man' between outlying districts and the party's Belfast based leadership.
He was a regular visitor to rural constituencies including north Antrim where Sinn Féin was working to build a popular base in the years after the 1998 Good Friday Agreement.
Former Moyle and Ballymena Sinn Féin councillor, Monica Digney encountered Mr Donaldson while serving as an elected representative in the area.
"Nobody knew what he did, he just sat there and didn't say much and offer much," she said.
"He would have just sat and listened."
Mr Digney said Mr Donaldson was active in north Antrim at the same time as fellow MI5 agent Denis McFadden, who was exposed as a suspected agent in 2020 after an MI5-led sting targeted the New IRA.
Ten people are currently facing charges linked to the case.
McFadden was a former member of Sinn Féin and chair of its North Antrim Comhairle Ceantair before.
He was later on the fringes of several republican groups before joining Saoradh.
Ms Digney said she suspected that Mr Donaldson "brought in McFadden."
In the late 1980s he was sent to the US to help reorganise republican structures there.
It later emerged that prominent Irish American and former Noraid director Martin Galvin raised concerns about Mr Donaldson's actions with senior republicans on several occasions.
Mr Donaldson was also a regular face on the Garvaghy Road during the Drumcree parading dispute in Portadown, Co Armagh, in the late 1990s.
At the time he was arrested in 2002 he described himself as a Sinn Féin Group Administrator in Parliament Buildings.
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