A jury of eight men and four women acquitted the man unanimously after deliberating for 90 minutes, the Irish Examiner reported.
O'Connell's comments drew condemnation from women's rights groups and sparked calls to change the way rape trials are conducted in Ireland.
On Tuesday, protests reached Ireland's parliament, the Dail, where Ruth Coppinger, a member for Dublin West, produced a blue thong during a debate.
"It might seem embarrassing to show a pair of thongs here... how do you think a rape victim or a woman feels at the incongruous setting of her underwear being shown in a court," she asked fellow parliamentarians.
Coppinger led a protest march through Dublin city centre under the slogan "end victim blaming in the courts".
Meanwhile, about 200 protesters marched through the centre of Cork to lay underwear on the steps of the court where the trial had been held.
Noeline Blackwell, the head of the Dublin Rape Crisis Centre, told the Irish Independent: "The reference to the girl's underwear and the assumption and inference that the jury was being invited to draw - that because she was dressed like that she was asking for sex - does not surprise us.
"It comes up very, very regularly how someone was dressed, the amount of drink they had taken, why they hadn't screamed if they were in trouble.
"These kind of mythologies and stereotypes around rape come up again and again in court cases, because the defence to rape is that the sex was consensual.
"So anything the defendant can do to suggest there was consent will be used," she said.
Charlie Flanagan, Ireland's minister for justice and equality, said he was open to the notion of changing the law on the conduct of rape trials but he would wait for the findings of a review group before making a decision.
"If there's legislative change necessary then I'd be very keen and I'd set about doing that," he said.
"But I believe it's more about the practice and procedure, particularly in the case that you mentioned," Flanagan told the Irish Examiner.
"I don't like to comment on individual cases, but it was a woman barrister who posed the question in that particular case."