The Green Party has said that in the past some of its activists may have posed as members of a polling company to canvass the opinion of voters.
It said activists may have passed themselves off as pollsters a decade ago - but insisted this is not something that the present day party approves of or would ever engage in.
In a statement the party said: "When we initially asked around internally yesterday it appeared that no-one in the party had ever engaged in using volunteers to carry out polling using a false company name.
"However, it later emerged that there may have been some isolated incidences of this taking place in some constituencies over a decade ago."
The Data Protection Commissioner has confirmed it has written to Sinn Féin about the practice of party activists posing as pollsters.
Similar letters are also expected to be sent to Fine Gael and Fianna Fáil today, following admissions by both parties that they also engaged in the practice.
This is an embarrassing turn of events for Irish politics, although the parties involved say that the practice of activists posing as pollsters was discontinued some years ago.
Sinn Féin has defended the practice, saying it was a common political exercise; the anonymity of voters was always maintained and the information was only used for polling purposes.
Both Fine Gael and Fianna Fáil confirmed they had employed a similar practice in the past - although Fianna Fáil discontinued the practice several years before Fine Gael.
Other political parties like, the Labour Party and the Social Democats have denied ever using such a system.
Cian O'Callaghan of the Social Democats said those involved must have had something better to do than deceiving voters.
Taoiseach and Fianna Fáil leader Micheál Martin said said a small number of party members were used to supplement the work of polling companies prior to 2007.
Speaking on Newstalk, he said it was not good practice and had been dispensed with since he became party leader.
Mr Martin said he did not know if such activity broke the law, but said no fake IDs were used or personal data taken or recorded and the party now uses professional companies for polling.
Fine Gael confirmed that some volunteers carrying out polling in the past had fake polling cards for a non-existent polling company.
"All Fine Gael polling from 2016 onwards have been carried out by commercial research companies and independent contractors," a spokesperson confirmed.
"Prior to 2016, the majority of polling would have been carried out by commercial research companies and independent contractors.
"Occasionally, members, who were paid or volunteered, carried out polling in constituencies.
"On occasion, they did not correctly identify where they were from and when asked, some would have replied by reference to a non-existent polling company and had business cards in support of that.
"This should not have happened," the spokesperson added.
"All polling data was gathered in an anonymous format and not retained. Unlike other parties, Fine Gael does not have a central database of voters.
"Fine Gael will respond fully to any queries from the Data Protection Commissioner."