The law firm says it relates to a tweet by Senator O'Riordain
One of the defendants in the recent Belfast rape trial is set to take legal action against a member of the Seanad.
Paddy Jackson was acquitted of all charges against him.
Senator Aodhán Ó Ríordáin posted commentary on his Twitter account following the verdict on Wednesday.
A statement from lawyers representing Mr Jackson say they are to issue notice of intention to sue Senator Aodhán Ó Ríordáin for "defamatory comments made by him in the immediate aftermath of the jury's verdict".
KRW Law says the legal action relates to a tweet sent to a number of other persons before it was eventually taken down.
It adds Mr Jackson "now finds himself resorting to the Civil Courts in order to seek protective action".
KRW Law says it will not hesitate to repeat similar legal action against anyone who deliberately or otherwise attacks its client.
Senior associate of KRW Law solicitors, Marie Hans, says: "I can confirm we have issued pre-action libel correspondence against a named Senator in the Republic of Ireland.
"The legal action related to a tweet sent to a number of others persons before it was eventually taken down."
She adds:"We will not hesitate to repeat similar legal action against anyone who deliberately or otherwise, sees fit to attack our client. We are examining carefully every item of social media commentary which seeks to challenge the integrity of the jury's full endorsement of our client's innocence.
"High Court proceedings will issue shortly in both Belfast and Dublin."
Seantor Ó Ríordáin was not available for comment at original time of publication.