Taoiseach Leo Varadkar has said some Sinn Fein politicians "appear to live way beyond their means" and has called for an investigation into the party's finances.
r Varadkar said questions surrounding Sinn Fein's political funding are one the main reasons why he will not engage in government formation talks with Mary Lou McDonald.
Writing in the Sunday Independent, he said: "Sinn Fein public representatives claim to earn only the average industrial wage, yet their salaries are not returned to taxpayers or donated to the party." He added: "Many appear to live way beyond their means," before calling for an investigation into their finances.
Mr Varadkar's intervention comes as Sinn Fein continues to surge in the opinion polls. Ms McDonald's party is up a massive 10 points to 35% with Fianna Fail (20%) and Fine Gael (18%) both trailing behind them.
The Sunday Times/Behaviour and Attitudes opinion poll also has Ms McDonald as the country's most popular political leader.
Her personal satisfaction rating is up 13% to 53%, while Mr Varadkar (27%) and Micheal Martin (31%) have both dropped points.
Yesterday, Mr Varadkar insisted there are "fundamental differences" between Fine Gael and Sinn Fein which would make them "incompatible" in government.
He accused the party of failing to embrace "constitutional democracy" and refusing to recognise the Republic as a "legitimate sovereign State".
He said Sinn Fein "operate differently" when it comes to political fundraising.
The acting Taoiseach said the party holds "lavish" fundraising events in America and money is then "routed through Northern Ireland".
He said the party bought office space using "front companies" and said Sinn Fein MPs collected £4.5m from Westminster without taking their seats.
"These irregularities cause us real concern and deserve further scrutiny and a proper investigation. Sinn Fein owes it to all of us to clear up these matters once and for all," he added.
A Sinn Fein spokesperson said Mr Varadkar's comments were a "disgrace" and beneath the Office of the Taoiseach.
"These comments are completely fabricated and this is outrageous mud-slinging on the part of the caretaker Taoiseach who is clearly struggling to get to grips with the election results and this weekend's opinion poll," he said. "These disgraceful and baseless comments discredit the office of Taoiseach and Leo Varadkar really should reflect on them," he added.