The £1.5m (€1.65m) donation left to Sinn Féin by a mechanic who lived in a mobile home has been raised with the UK's National Crime Agency (NCA).
William E Hampton left an estate of almost £2.6m (€2.87m) in the UK when he died last year and Sinn Féin disclosed the share it has received to the UK's Electoral Commission.
However, Sinn Féin's political opponents on both sides of the Border have now raised questions about the mystery donation.
Mr Hampton was last night described as a "recluse" and a "frail individual" by another beneficiary of the will, former 'Private Eye' journalist Paul Halloran.
London-born mechanic Mr Hampton died in his home in Wales at the age of 82 in January 2018. He was "of no fixed abode" at the time he made the will in 1997.
The majority of the estate was handed to the executors and trustees - Joe Cahill and Dessie Mackin - who were Sinn Féin's national treasurers. The money was intended to "cover election expenses, to fund Sinn Féin offices and advice centres and to aid republican prisoners".
Traditional Unionist Voice leader Jim Allister raised the donation with the UK's National Crime Agency (NCA).
Mr Allister sent a letter and a copy of the will to the NCA and asked if it has looked into the source of the funds.
Separately, Fine Gael Senator James Reilly urged Sinn Féin to return the donation to Mr Hampton’s estate.
He said it would break Irish political spending laws if any of the money was used in the south and Sinn Féin leader Mary Lou McDonald should confirm that none of the money will be spent here.
Mr Reilly said: “The honourable thing for Sinn Féin to do would be to return the money to the estate”, and added that this would prove that Sinn Féin is a “legitimate law-abiding party”.
Sinn Féin did not respond to Irish Independent requests for comment last night on Mr Allister’s letter and Mr Reilly’s remarks.
The party previously said Mr Hampton came from a wealthy family background.
Legal documents show he insisted that on his death the majority of his remaining assets in “Ireland, England, Singapore and New Zealand” should be bequeathed to Sinn Féin.
Journalist Mr Halloran – who was left £1,000 by Mr Hampton – said the mechanic approached ‘Private Eye’ in the 1990s because he believed he was being persecuted by a firm of accountants that acted for his father’s business.
He said he was a “recluse” who was “not an unintelligent person” but who “lived on his own and moved around a lot”.
Mr Halloran said there was no story as the accountancy firm had merely been trying to pay dividends to Mr Hampton.
He said he was in sporadic contact with Mr Hampton in the years following his approach to ‘Private Eye’.
Mr Halloran learned that he was included in the will when a copy was sent to him in 1997 but has not heard from him since.
He said he tried to contact Mr Hampton through a firm of solicitors in Dundalk who had been acting for him to ask him not to bequeath him any money.
Mr Halloran does not know if Mr Hampton ever got the message.
He said he only learned of Mr Hampton’s death two months ago when he was contacted by solicitors over the £1,000 (€1,100) bequest.
He said he had no knowledge of Mr Hampton’s links to Sinn Féin.
Mr Halloran recalled one phone conversation he had with Mr Hampton: “He said to me...‘you know you’re one of the few people in my life that’s ever treated me with kindness’, which will tell you a lot about the man himself.
“It’s very sad.”
Mr Hampton was of “no fixed abode” when he wrote his last will and testament and was living in a mobile home in Ireland at the time.