A senior garda has been released without charge following his arrest in connection to an probe into the unlawful disclosure of information.
The garda was arrested after he attended a garda station by appointment yesterday morning.
He was questioned as part of the investigation into the unlawful disclosure of information and was held overnight, when his period of detention was extended by 12 hours.
Meanwhile, the National Union of Journalists has raised concerns about what it described as "a number of unusual aspects" in relation to the arrest of a senior garda .
The investigation follows an inquiry by the former Children's Ombudsman Emily Logan into the removal by gardaí of two children from their families in Athlone and Dublin in October 2013.
The report found that while the Health Service Executive could not be ruled out, the release of details in relation to one of the children probably came from someone within An Garda Síochána and may have been an offence.
The officer was arrested at 7.30am yesterday after he had been requested to attend a north Dublin garda station in the company of his solicitor.
He was questioned in connection with breaches of the Garda Síochána Act into an alleged leak to the media.
The secretary of the National Union of Journalists in Ireland Seamus Dooley last night said it is monitoring the situation carefully.
"Journalists must be allowed to operate in the public interest and we would be greatly concerned at any development which might lead to further barriers to the release of information in the public interest," he told the Irish Independent.
It is understood that the officer concerned was authorised by the then Garda Commissioner Martin Callinan to speak to the media at the time the children were removed from their families.
His successor, Nóirín O'Sullivan launched the inquiry last July following the publication of Ms Logan's report into the use of legislation by gardaí in Tallaght and Athlone to remove the children and place them in care. The children were subsequently returned to their families.
A Garda Chief Superintendent and a Detective Superintendent have been carrying out the inquiry.
It is understood that a computer and telephone were examined for evidence and the officer's office was searched.
The officer, who has been suspended from duty, is well respected in the force and the decision to make an arrest was greeted with surprise by his colleagues yesterday.
He is the most senior garda officer ever to be arrested as part of a criminal investigation.
The Garda Press Office would not comment on the arrest except to confirm that the investigation into the disclosure of confidential and sensitive information was ongoing.
Ms Logan's found that the details contained in an article about one of the children "on the balance of probabilities" came from someone within an Garda Síochána.