Leo Varadkar said Jim O'Callaghan could have recused himself from representing Gerry Adams.
"You can recuse yourself from a case if you declare a conflict of interest," he told Clare FM.
"Now isn’t it a conflict of interest that the Fianna Fail justice spokesperson, who was legal advisor to Micheal Martin before he was justice spokesperson, is representing the (then) leader of Sinn Fein, the former leader of Sinn Fein in the courts?
"I’m pretty sure, if he wanted to, he could have found a way to recuse himself. "
Mr Varadkar added: "There is a lot of people now in Fianna Fail - candidates all over the country - who have indicated in the past couple of weeks that they are open to coalition with Sinn Fein.
"I think a vote for Fianna Fail is a vote for Sinn Fein in this election."
O'Callaghan: I do no discriminate against people as a barrister because of their politics
Fianna Fáil's Jim O'Callaghan said it would be a breach of his profession's code of conduct if he refused to represent someone legally because of their politics.
It is after it emerged Mr O'Callaghan was hired by former Sinn Féin leader Gerry Adams to act in a defamation case.
Barrister and TD Jim O'Callaghan has been heavily critical of Sinn Féin during the election.
But the Fianna Fáil candidate said he separates his private profession and his politics.
"I represented thousands of people, I've been a barrister for 25 years," he said.
"I've represented Fine Gael TDs and ministers, Fianna Fáil TDs and ministers, Labour politicians, people from the SDLP, Sinn Féin, unionist parties in Northern Ireland, prominent businessmen and prominent trade unionists.
"I do no discriminate as a barrister between people who come to me looking to be represented, nor could I.
"I would be breaching the code of conduct of the Bar of Ireland if I decided I was not going to represent that person because I don't like their politics."
However, Fine Gael's Regina Doherty said the Bar’s code of conduct allows for barristers to “step away from a case” where there is a conflict of interest or likelihood of a conflict of interest arising.
“But what we have here is Fianna Fáil want us to do what they say and not what they do,” she told RTÉ’s Today with Sean O’Rourke.
“You have to be able to trust in what people say. So before people go to the ballot boxes on Saturday and make that decision I think it is fair for them to know if they can trust what Fianna Fail says.
“And is it fair for Jim O‘Callaghan to come out and lambast Sinn Féin for their policies or for their past political manoeuvres and still be able to take their coin by doing their work.”
Ms Doherty said she was not just singling out Mr O’Callaghan, adding that a number of Fianna Fáil TDs had suggested a coalition with Sinn Féin was possible.
On Mr O’Callaghan not being a TD when representing Mr Adams in 2015, Ms Doherty said: “Jim was desperately attempting to be a TD in 2015 and always knew he was going to be on the ticket.”
“It is about whether people can trust Fianna Fail when they say they won’t go into government with Sinn Fein,” she said.
Ms Doherty said there are “decent people in Sinn Féin” and accepted she worked with the party during her time as Government Chief Whip.
But it was the party’s policies which were “absolutely unacceptable” to her, she added.
“You have to talk the talk, and walk the walk.”