A man in his 30s was detained in Laois on Wednesday and questioned in the midlands.
A Laois senator says she was aware of the criminal convictions of a Sinn Fein supporter who is now a terror suspect.
It comes after a man was arrested last week over a far-right plot to attack a mosque.
The man has previously admitted to dealing cocaine and had several non-drug related convictions.
Sinn Fein's Maria McCormack says she met him through his partner when she was working as a palliative carer.
She says she was aware of his historic convictions, and drug arrest, but was assured he had turned his life around.
Ms McCormack is describing the news of his arrest in relation to terrorism activity as profoundly shocking.
She had twice signed the man in to Leinster House as a guest.
Sinn Fein says he has had two applications to join the party rejected in the last year.
He has also met Mary Lou McDonald and has been seen erecting posters for party events.
Maria McCormack's statement in full:
"I am deeply shocked at the arrest and charging of people in connection with a far-right plot to kill and maim. Those feelings are shared across Laois Sinn Féin and in the wider community. Garda investigations exposed a sinister, violent threat to innocent people and undoubtedly saved lives. They are to be commended for their actions.
"A female member of Sinn Féin has been expelled from our party following a raid on the home of her, and her partner, and his arrest in connection with the Garda investigation into the most serious and repugnant of actions by a far-right extremist group in Laois.
"I met the woman in 2023 when I worked in palliative care and I was a carer to her terminally ill mother. I am shocked and angry at what has happened.
"I also want to address the issue of the local man who was arrested. I met him when I was providing palliative care to his partner’s mother. He applied to join the party during the election campaign in November 2024 and again the following year but his membership application was not accepted and I supported that decision.
Any interaction he had with Sinn Féin was informal and solely through his partner, who was a member at the time. I was aware of his historic convictions and drug arrest but was assured that he had turned his life around. I accepted that.
"At no time did I have any knowledge whatsoever of the concerns now under Garda investigation, nor was there ever any indication of involvement in far-right extremism or violence.
"The actions now under investigation are utterly shocking, and they bear no resemblance to the individual as he presented himself to me.
"What has emerged is deeply distressing, and I fully commend the Gardaí for their decisive work.
"I abhor the violence and racism of the far right. They thrive on hatred and fear. Over recent years they have abused, threatened and attacked innocent people, including Sinn Féin representatives and members. Some are in jail due to death threats they have made against the Sinn Féin leader.
"We will not allow our party to be exposed to any far-right elements who want to disrupt our activism. We continue to work closely with the Gardaí, who are vigilant in the face of these threats.
"As this matter is subject to serious legal proceedings, I cannot comment further at this time."
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