Latest figures show its support has reduced from 21% to 18% in the Red C poll and from 24% to 21% in a separate Millward Brown analysis.
The Red C poll sees Fine Gael stable on 28%, Fianna Fáil rising one percentage point to 20%, independents/others gaining five points to 23%, Labour dropping three points to 7%, and Renua static on 1%.
Similarly the Millward Brown poll places Fine Gael up four percentage points on 29%, Fianna Fáil up four to 23%, independents/others unchanged on 20%, Labour down two points to 6%, and Renua on 1%.
The drop in Sinn Féin’s popularity will be of particular concern for the party given that it dovetails with recent controversy within the organisation.
Last week, Cobh-based Cork County councillor Kieran McCarthy was expelled while Mallow- Kanturk colleague Melissa Mullane was suspended for 12 months.
While the party has asked both councillors to resign their seats due to the affair, they are yet to do so.
Dozens of their supporters have said they will resign from the party over what happened, a claim that Sinn Féin headquarters insists has not yet taken place.
The party is due to hold emergency “information” meetings with its grassroots members in the constituency over the coming days.
The local TD at the centre of the dispute, Ms McLellan, is not expected to attend.
Speaking on RTÉ programme This Week, Ms McLellan said she will contest the seat at the next general election, if selected — with Cork East the only constituency in the county that Sinn Féin has yet to confirm who will run for it — despite admitting the past few days have been “difficult” for all concerned.
Weekend reports suggested Midleton-based suicide-prevention campaigner Pat Buckley may seek the nomination instead due to the ongoing bad feeling between Ms McLellan and some grassroots members and fears this could hamper the party’s election bid.