Gerry Adams likened to Mugabe in attack by Fine Gael rival Hayes
Sinn Fein is answerable to the republican criminal underworld with "their boilersuits and balaclavas", an Irish MEP claims.
Brian Hayes, Fine Gael's director of elections, also compared Sinn Fein leader Gerry Adams to President of Zimbabwe Robert Mugabe.
In the strongest attack yet by the main government party on Mr Adams, Mr Hayes questioned who really runs Sinn Fein.
He said: "Sinn Fein presents itself as something new despite the fact Mr Adams has been around as long as Robert Mugabe."
Mr Hayes's comments, made during a speech at the Fine Gael ard fheis in Citywest, were met with roars of approval from up to 3,000 party delegates.
The MEP also described Mr Adams as "that well-known economic guru from west Belfast" and said if the coalition government in the Republic had followed his advice the country would be facing 30 years of austerity.
Taoiseach Enda Kenny made no mention of Sinn Fein in his speech from the conference but Mr Hayes used his contribution about 10 minutes before Mr Kenny's televised address to rouse the crowd.
"This party isn't fooled by Sinn Fein. We know what they are. The Irish people don't want Sinn Fein next or near the government of this Republic," he said.
"But the crucial question for Sinn Fein is, who do they answer to? Is it the people or the republican criminal underworld - with their boilersuits and balaclavas?"
He also hit out at Fianna Fail and referenced their controversial attack poster on Fine Gael's health record, saying they "can't even get a proper poster campaign right these days".
A major report into paramilitary activity last year suggested the IRA Army Council still exists and oversees both the Provisional IRA and Sinn Fein "with an overarching strategy".
The independent panel appointed by the British Government stated that "the structures of PIRA remain in existence in a much reduced form" but those that remain include "a senior leadership, the "Provisional Army Council" and some "departments".
It warned individual members of paramilitary groups represented "a threat to national security".
But the three-member panel also found the leaderships of the groups were "committed to peaceful means to achieve their political objectives".
While not actively recruiting or rearming, it said IRA members believed the Army Council "oversees both PIRA and Sinn Fein with an overarching strategy", although it has a "wholly political focus".
Mr Mugabe, who turns 92 on February 21, is the only President Zimbabwe has ever known.