Senator Lorraine Clifford-Lee has apologised for tweets she posted in the past in which she used the words "knacker" and "Traveller" in a derogatory manner.
Other Twitter messages said celebrity Kim Kardashian had a "fat arse", the Fianna Fáil politician also told how a "black Brazilian dwarf with ginger hair" sat beside her on a bus.
Ms Clifford-Lee (38) is a candidate in the Fingal constituency by-election which will be held on November 29, and has branded the publicity surrounding the tweets as a smear campaign.
The controversial social media posts will be a headache for Fianna Fáil leader Micheál Martin.
Attention was drawn to the historical tweets on social media over the weekend and Ms Clifford-Lee last night apologised for any offence caused by the posts and admitted they were wrong.
In July 2011 she tweeted: "Just doing up inventory of items in my car when it was stolen by some knacker. I think I'm going to cry."
In June the same year she had responded to a tweet about fashion, which is now no longer available, with: "I think some pieces are cute but others look like something from a Traveller wedding."
In October 2011, Ms Clifford-Lee tweeted: "Was on Dublin Bus for the first time in about a year yesterday. Forgotten how much fun it is. Black Brazilian dwarf with ginger hair sat beside me."
Then in June 2012, she wrote: "Fat arse Kardashian is in Dublin apparently with her fella."
Ms Clifford-Lee is the Fianna Fáil Seanad spokesperson on justice and equality, as well as children and youth affairs. Her Twitter biography describes her as a solicitor, mother, and feminist.
Asked by the Irish Independent for a comment on her controversial tweets, Ms Clifford-Lee issued a statement.
She said the tweets referred to are from as far back as 2011 and were posted long before she became involved in electoral politics.
"I want to be absolutely clear, the tweets were wrong and I apologise for any hurt or offence caused by them," she said.
"While it does suit some people to take my tweets out of context for their own right-wing agenda, I hope that the people of Dublin Fingal will not be distracted by personal smear campaigns such as this.
"Anyone who has worked with me since my election to the Seanad three years ago can confirm the role I played in promoting progressive policies.
"I also actively supported the marriage equality referendum and the campaign to repeal the Eighth Amendment."
She said she wanted the by-election campaign to focus on "the very real issues that people face every day".
Ms Clifford Lee listed these as solutions to transport, housing, health, childcare, climate change and protecting communities from crime.
She was elected to the Seanad in 2016 following an unsuccessful bid for a Dáil seat in the same constituency in that year's general election.
Fianna Fáil has high hopes she will join Dublin Fingal party colleague Darragh O'Brien in the Dáil in this month's election.
A row has erupted between the Road Safety Authority (RSA) and Fine Gael by-election candidate Verona Murphy in the wake of remarks she made after the Essex tragedy that saw 39 migrants found dead in a lorry.