Dublin City Councillor Noeleen Reilly has told RTÉ News that she has resigned from Sinn Féin.
It follows a decision by the party on Saturday to suspend her for six months after it investigated a complaint from another councillor.
Sinn Féin TD Dessie Ellis was censured by the party for media comments he made about Ms Reilly.
She said she was shocked by the decision to suspend her and claimed that several recent bullying complaints she made have not been examined by the party.
I am resigning from Sinn Féin with immediate effect.— Cllr Noeleen Reilly (@NoeleenReilly) February 5, 2018
Today I hand in my resignation from Sinn Féin after 17... https://t.co/4JHBssMtkK
Ms Reilly described her treatment by Sinn Féin as "torture" since she first made bullying allegations in 2014 and said she had no choice but to resign.
She said every time she went to the party about the alleged bullying she was "either ignored or an angle was found to blame me for their treatment.
"Unfortunately, I was just up against the wrong people and this was never a battle I was going to win."
The councillor has informed Dublin City Council that she is now an Independent member.
However, she has not written to Sinn Féin, saying there was little point.
In response, Sinn Féin issued a statement, saying the party's ard comhairle made the decision to suspend Ms Reilly for orchestrating a campaign against a fellow female party councillor on social media.
It said the party is now requesting that Ms Reilly hands the Sinn Féin Dublin City Council seat back to the party. "Sinn Féin will not tolerate bullying," it added.
"In relation to Councillor Reilly's allegation that she was assaulted, the party told Councillor Reilly on numerous occasions that she should report the alleged incident to An Garda Síochána," the party said.
Responding to the Sinn Féin statement, Ms Reilly said that she was never involved in a bullying campaign against another councillor.
She also said that she would not be handing back her seat on Dublin City Council to Sinn Féin.
"No I won't. And they have some cheek after what they allowed happen to me," she said.
Councillor Reilly said she had not made a decision as to whether or not she would run as an independent candidate in Dublin North West at the next election.
The councillor's resignation brings to more than 15 the number of party representatives who have resigned in recent years amid claims of bullying.