Rugby player Paddy Jackson, who was acquitted, had threatened to sue him.
The tweet, which Mr Ó Ríordáin subsequently deleted, praised the complainant and criticised the defendants. It was tweeted under the hashtag #Ibelieveher.
“My tweet of 28 March, concerning the Belfast rape trial, was not designed or intended to suggest that either Paddy Jackson or any of the other accused men were guilty or that the jury got it wrong,” said Mr Ó Ríordáin.
“I apologise for any suggestion to the contrary. I accept that I was not privy to all the evidence put before the jury during the trial. I will not be making any further comment.”
The threat to sue the former government minister — and to repeat similar legal action “against anyone, who deliberately or otherwise, sees fit to attack our client” — led to hundreds of people tweeting under the hashtag #suemepaddy.
Meanwhile, a judge is to consider whether there is any legal basis for continuing reporting restrictions on the rape trial of Mr Jackson and fellow Ulster and Ireland rugby player, Stuart Olding.
Judge Patricia Smyth said she will hear legal argument on the matter this week.
Mr Jackson, aged 26, and Mr Olding, aged 25, were unanimously acquitted of rape. Mr Jackson was also acquitted of sexual assault.
Restrictions on the reporting of legal exchanges that take place in the absence of the jury usually fall away once the trial is over, as the issue of prejudicing jurors is no longer relevant.
A number of outlets that covered the marathon trial want restrictions imposed by Judge Smyth to be lifted.
The case has been listed for hearing on April 25, but lawyers for the press want it dealt with expeditiously.
Gerry Simpson, QC, representing a number of print and broadcast media, said: “This is a matter which we say should be dealt with very quickly.”
He said there is “no basis” for the continuation of restrictions.
Barrister Stephen Toll, representing Mr Jackson, said: “I do not think we have proper focus on the issues, because the media are refusing to say what applications they are wanting to rule on.
They can stand on their rights and that is their prerogative. We may have a concern about the convention rights [human rights] of the defendants and other persons that may be affected.
Mr Simpson said: “The media are not refusing to do anything. I have no idea, because I do not intend to ask the media. That’s the wrong question to ask.”
The lawyer added that the “only question” to ask is if there is a “lawful basis “ for the continuation of restrictions.
Judge Smyth said: “I agree that is the correct question that we have to address.”
Two other men were found not guilty of lesser charges connected to the alleged incident in June, 2016. Blane McIlroy, aged 26, was acquitted of exposure, while Rory Harrison, aged 25, was found not guilty of perverting the course of justice and of withholding information.
The case has been adjourned until tomorrow.