Commissioner O'Sullivan is due to appear before an Oireachtas committee tomorrow amid the latest Garda scandals
Fianna Fáil says it is unfair that every ordinary member of An Garda Síochana is under a cloud of suspicion because of the handling of the latest controversies in the force.
Pressure is mounting on the Garda Commissioner to step down amid the scandal surrounding breath tests and speeding convictions.
Last week, management confirmed that between 2006 and 2016 there were 147,000 motor offences where a court summons was issued instead of a fine. 14,700 of those cases resulted in a penalty being imposed by the courts.
Separately, Gardaí discovered a massive discrepancy in their recorded number of breath tests, with almost a million extra tests recorded.
Yesterday, the Tanáiste and Justice Minister Frances Fitzgerald strongly defended Nóirín O'Sullivan.
She told the Dáil last night that the Commissioner is the best person to lead the force.
Minister Fitzgerald told deputies: "The Garda Commissioner was appointed by an independent process.
"She came through that process, and was deemed to be the best person to lead the reform and to lead An Garda Síochana."
The Government has proposed an independently-run 'root and branch investigation' into Garda procedures.
Commissioner O'Sullivan, meanwhile, is due to appear before an Oireachtas committee tomorrow.
Sinn Féin has already published a motion of no confidence in the Commissioner, while Solidarity-PBP has also said it is in the process of putting forward a motion.
Fianna Fáil has maintained that it cannot yet express confidence in Noirín O'Sullivan, but will 'review its options' after tomorrow's committee meeting.
Speaking on Newstalk Breakfast this morning, the party's finance spokesman Michael McGrath said several questions remain unanswered.
He argued: "We are going to have to once and for all deal with the culture within the force.
"The fact that the Commissioner has been unable to tell us exactly who was responsible for this scandal means that there is a shadow of doubt and a cloud of suspicion hanging over every ordinary member of the Gardaí today - and that's not fair."