Sinn Féin and Fianna Fáil have confirmed that activists from the respective parties acted as pollsters and went door-to-door to gather information from people.
Leo Varadkar also confirmed that in the past Fine Gael acted similarly, using students and volunteers.
All parties have now said they do not use these practices anymore.
The Data Protection Commissioner (DPC) has written to Sinn Féin over the incident, and is likely to contact Fianna Fáil and Fine Gael as well.
Sinn Féin spokesperson on housing, Eoin Ó Broin, defended the practice from the party and claimed it was a common element of an election campaign.
Leo Varadkar also said that Fine Gael has not engaged in similar practices since 2016.
The DPC will likely contact all political parties involved in these practices.