Eoghan Harris' involvement in an anonymous Twitter account was uncovered when the account shared details of a united Ireland poll which had not gone to print.
The former columnist with the Sunday Independent had his contract with the newspaper terminated on Thursday after management discovered Mr Harris was contributing to the fake account which used the name Barbara Pym.
The connection between Mr Harris and the fake account was made after the account posted details of a poll commissioned by the paper on a united Ireland, the results of which had not yet been shared publicly but had been given to Mr Harris prior to publication, according to The Irish Times.
Alan English, editor of the Sunday Independent, said the content posted by the fake account, much of which was against Sinn Féin and nationalists, was a "betrayal of trust" and went beyond fair and reasonable comment.
Following the publication of the poll data on the fake account, the former senator was approached by management upon which he admitted to contributing to the Barbara Pym account.
Mr Harris confirmed this sequence of events to The Irish Times on Friday, adding that he thought the poll results were in the public domain prior to them being shared by the Pym account.
Twitter has since suspended the Pym account for infringing the site's 'platform manipulation' rules, also banning a further eight accounts said to be linked to the account which used the handle @barabarapym2.
Mr Harris said he was only involved in the Pym account, which he said was also operated by five or six other people whom he would not name, saying he did not want to "get them shot".
The Pym account often praised the work of Mr Harris and his colleagues while criticising other journalists.