Red C Research and Marketing has apologised after it emerged that seven replacement members of the Citizens' Assembly were improperly recruited.
The company said the seven replacement members were recruited by one of its employees "outside of the strict methodology" for choosing assembly members.
They were recruited by phone rather than being recruited by cold-calling households door-to-door and were identified as potential members through friends and family of the recruiter.
Red C said they took part in only one weekend meeting on 13 and 14 January this year, when members looked at "the manner in which referenda are held".
It has suspended the recruiter involved pending an internal investigation.
It said: "RED C is satisfied that this was an isolated incident, and fully stands over the integrity of its work for the Citizens' Assembly prior to this isolated incident.
"RED C remains confident that all members recruited prior to the meeting in mid January 2018 were recruited to a very high standard, as requested by, and agreed with, the Citizens' Assembly."
Chair of the assembly retired judge Mary Laffoy said she was satisfied the incident had no impact on the work of the assembly, which had 99 members and the chairperson.
She said: "I want to assure the public in the clearest possible terms, that I am satisfied that this is an isolated incident, and that it has no impact on the work of the assembly on previous topics.
"I can also confirm that the seven individuals in question have been informed of the position and as such will play no further part in the work of the assembly.
"While this incident is regrettable, I believe that the actions which we have taken and will take to address the situation, ensure that the robustness of our work and the transparency and openness in which it has been conducted, can be maintained."
However, the Pro Life Campaign has called for a "full and immediate audit of the recruitment process".