The Democratic Unionists have faced calls to disclose more details about their relationship with a pro-Union group that bankrolled their £425,000 Brexit campaign.
Political rivals have demanded answers after the DUP revealed its main donor was a little-known Great Britain-based group of business figures called the Constitutional Research Council (CRC).
The party had been under mounting pressure to reveal the name of the donor which enabled it to promote its Leave message on a UK-wide basis, including the purchase of a four-page Vote To Leave EU advert in the British Metro newspaper - a publication not circulated in Northern Ireland.
After Friday's disclosure, Sinn Fein's Conor Murphy said the affair "made a mockery" of DUP calls for transparency in politics before next week's snap Assembly election.
"It is still not clear who is in the group that gave the DUP £425,000, and what the group got in return for this huge donation," he said.
He added: "The public is entitled to expect integrity in the political process.
"These are key issues of accountability and transparency and the public have a right to know so they can make up their mind before they go to the polls next week."
The leader of the cross-community Alliance Party, Naomi Long, said questions remained over the DUP's Brexit expenditure.
She said the party should release a detailed breakdown of how the money was spent.
"Would the DUP have campaigned as vigorously for Brexit had they not received a large donation to do so?" she said.
"How does that set against the previous allegations by Arron Banks that they were essentially charging a fee to join a Leave campaign? All of those questions need to be answered and answered openly."
Mr Banks, a multimillionaire who poured millions of pounds into the campaign for the UK to leave the EU, previously claimed that the DUP demanded money to back him.
The DUP denied the allegations by Mr Banks in a book called The Bad Boys Of Brexit.
Ms Long said: "We still need full transparency so that people can make their own judgment as to whether parties are acting on their behalf as the electorate or on behalf of those with deep pockets and fat wallets."
The CRC is chaired by former vice-chairman of the Scottish Conservative Party Richard Cook.
DUP MP Sir Jeffrey Donaldson said: "It has been involved in Scotland, for example supporting unionism in Scotland, and it approached the DUP to support our campaign during the referendum because it supports unionist causes in the United Kingdom."
The names of donors in Northern Ireland are automatically withheld due to Troubles-era rules which sought to protect their security. That had prompted speculation that prominent Leave campaigners were using the DUP to support the Leave campaign without the need to publish their names.
Sir Jeffrey rejected that suggestion.
The Lagan Valley MP said the CRC had donated to the DUP because the main Leave campaign had already reached the limit of its spend.
He outlined the details hours before the Electoral Commission published details of campaign spending on the EU referendum.
"We wanted to be involved in the referendum at a national level," he said.
"We recognise that the population of Northern Ireland is 3% of the United Kingdom. This referendum wasn't going to be won or lost in Northern Ireland, it was going to be won or lost on a national basis, and that's why the DUP, being a unionist party, decided to participate in the national campaign and we registered as such and, after we registered, we were able to raise this money and spend it all on campaigning both on a national level and indeed we spent some of it in Northern Ireland."