Sinn Féin and Fianna Fáil have said that activists posed as members of a polling company and went door-to-door to canvass the opinions of voters.
Fine Gael leader and Tánaiste Leo Varadkar confirmed his party did something similar using volunteers and students.
All three parties said they have stopped using the practice.
The Data Protection Commissioner (DPC) has confirmed it has written to Sinn Féin about party activists posing as pollsters.
It is expected that the DPC will also send similar letters to Fine Gael and Fianna Fáil.
In a statement this evening, Fianna Fáil said: "Since 2007, the party has outsourced polling to private, independent providers.
"Prior to 2007, we did on occasion use party members to undertake polling and on those occasions they did pose as market researchers.
"This was to supplement private providers also used. This practice was ended around 2007."
Sinn Féin housing spokesman Eoin Ó Broin defended the practice, which has now been abandoned.
Mr Ó Broin was commenting earlier on a report in the Irish Independent that quoted an internal party manual instructing supporters on how to pose as researchers.
The document dates back to 2015. It is understood the party now uses professional polling companies to gauge the opinion of voters in constituencies.
Mr Ó Broin said that Sinn Féin engaged in the practice from 2010 to 2016 and described it as "a common part of election practices".
Speaking on RTÉ's News at One, Mr Ó Broin said he did not believe that those involved were "lying" to people during the process.
Members of the party "were collecting polling data" in his own constituency of Dublin Mid West on doorsteps in advance of the 2016 election, Mr Ó Broin added.
"If I had the resources, and employed a private company, it wouldn't have been declared that they were being employed to do the poll on behalf of Sinn Féin.
"I just want to stress that the polling activity we were doing was no different to any of the polling activity whether by professional companies, or the informal polling that is done by political parties on a regular basis.
"It was fully anonymised, and the information was used solely for the purposes of political polling, which is a commonplace practice."
When asked if members were supplied with fake identification badges, Mr Ó Broin said: "They would have had ID badges that would have had their real names and photographs and the name of the marketing research company."
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Responding to the earlier comments from Mr Ó Broin, Mr Varadkar said that Fine Gael had also previously engaged in a similar practice.
Speaking to RTÉ's Drivetime, Mr Varadkar said the practice was discontinued by 2016.
"Quite frankly, yes," he said, when asked if his party had engaged in similar practices.
"This isn't something that we've done since 2016 or even before that, but certainly prior to that, we would have done something similar. They’d be either volunteers or students.
"Volunteers would have been asked to do surveys door to door, or students would have been paid to do it, and it would have been done on a similar basis anonymised for the purposes of polling.
"That practice has been discontinued certainly [in] 2016 2017, maybe as far back as 2014, but certainly not in the last five, six, seven years."
Reporting by David Murphy and Paul Cunningham
