The Sinn Féin President Gerry Adams has said the DUP needs to explain the background to the donation of about £435,000 that it received in last year's EU referendum campaign.
The money from the Constitutional Research Council was spent on pro-Brexit advertising throughout the UK.
The group of pro-union business people is led by a Conservative party member.
About £425,000 of the funding was spent mainly on advertising in Britain. The rest went to DUP party funds.
Speaking at an anti-Brexit rally in west Belfast on Saturday, Mr Adams said the DUP needed to provide more answers.
"The DUP acted as a funnel for £425,000 coming from a Scottish Tory, which had an impact on the outcome.
"They need to explain how they got themselves into that position."
How the £425,000 was spent
- £282,000 on advertising in Metro newspaper in support of Brexit
- £99,616 on promotional material
- £32,750 with Canadian IT and consultancy firm
- £10,823 spent in Northern Ireland
Speaking on the BBC's Good Morning Ulster programme on Friday, DUP MP Sir Jeffrey Donaldson. stressed it was "not a Northern Ireland referendum, but a UK-wide referendum".