A group of Fianna Fail supporters opposing the programme for government is under investigation by the Data Protection Commission (DPC) following complaints people were signed up to Fairer Future without their consent.
Elected and unelected party members have complained about receiving emails from the group thanking them for their support.
The complainants claim they had never pledged to support Fairer Future - which calls for a debate on the programme for government and a no vote in a ballot of Fianna Fail members on the deal with Fine Gael and the Green Party.
In some cases, the complainants claim they received emails to parliamentary accounts and their personal email addresses.
Sinn Fein TD David Cullinane was also signed up to Fairer Future by someone who filled in the online application form using his Oireachtas email address and assigning 'Shinner' as his first name. He then received an email from Fairer Future saying: "Hey Shinner! Well done for signing up."
Members of Fianna Fail's own parliamentary party claim to have also received the unsolicited emails.
Members of the party's front bench have been critical of Fairer Future's supporters, with Galway TD Anne Rabbitte saying: "These lads aren't progressive politicians and they deserve a slap around the ankle."
Fairer Future said it has now introduced a "double verification system" where a person can confirm they intended to pledge support or have their details deleted from the group's records after receiving a confirmation email. It says the feature should also "deter those maliciously trying to disrupt and distort our campaign".
A spokeswoman for the DPC said: "We have received complaints in relation to this and are assessing the issues raised."
Cork county councillor Deirdre Kelly, who is supporting Fairer Future, said: "Maybe it should have been more difficult to register initially.
"The last thing we have had time to do is pick random people and sign them up out of the sky.
"I registered and it was really easy but my details are online, so it would have been easy for someone to get my details and register me."
Roscommon county councillor Orla Leyden believes the group is being treated unfairly.
"This is a group of grassroots people within the party who were opposed to a coalition with Fine Gael before the election and we still are opposed to it. The party leadership was opposed to it during the general election," she said.
"We are just maintaining that line. We have extreme integrity and are standing by what was said."
Fianna Fail headquarters moved to distance itself from the group last week, saying the party operates "to the highest standards in terms of data protection and adherence to GDPR".
"This group is not an official unit within Fianna Fail," a spokeswoman said.
"It has had no communication or involvement with the national party, nor has any data been sought, or provided by the party to it. If it was sought, it would not be provided. Any investigation into this grouping and its activities is a matter entirely for the DPC and the people involved in this grouping."