An Irish citizen captured by Kurdish-led forces in Syria was regarded as a "serious player" by gardaí investigating the activities of suspected jihadis when he lived here.
The man, who is 45, held an Irish passport and lived in south Dublin for a couple of years.
He was closely monitored by members of the Garda Special Branch and its Intelligence and Security section until he left this country with his family in 2013 and travelled to Syria.
The suspect was named last night as Alexandr Ruzmatovich Bekmirzaev, who is thought to have originally come from Belarus.
He worked in a fast-food business in Dublin, but was suspected by anti-terrorist Garda units as being a key player in providing logistical support to activists linked to Islamic State (Isil).
Bekmirzaev was one of half a dozen people who left here to fight for Isil and are believed to be still alive and operating in Syria.
He was regarded by the gardaí as a "person of interest" and was described by one officer last night as "a lot more than a sympathiser".
The officer said Bekmirzaev had been "hanging around with a number of people who we regarded as the main [Isil] players in this country and who had been top of our monitoring list".
Bekmirzaev was also said to be "of interest" to other agencies. He was suspected of being heavily involved in providing logistical support to Isil fighters. This generally involved producing false or forged documentation, including identity papers, and fundraising for those involved with Isil in Syria and Iraq.
He was arrested by the Kurd-led Syrian Democratic Forces (SDF) as he and four others, two US citizens and two Pakistanis, fled towards a region controlled by the Kurds and US forces to avoid capture by Syrian government forces.
The captures were made on December 30 but details of the operation only became known here publicly yesterday.
The SDF said they arrested the five suspects in Deir al-Zor during an operation to protect a large convoy of civilians, who were being targeted by Isil.
The other four were named as Warren Christopher Clark (34), also known as Abu Mohammed al-Amriki, from Houston, Texas; Zaid Abed al-Hamed (35), also from the US; Fadel al-Rahman (48), from Lahore, Pakistan; and Abdul Adhim Rahjoud (19), from Sialkot. The SDF said all five had been arrested for alleged membership of Isil.
It said Isil was now cornered in a small area after being cleared from large swathes of territory it once held, and was suffering heavy losses.
The threat level posed by international terror groups here remains moderate and the biggest danger is seen as likely to come from a "lone wolf", somebody who has been radicalised and operates alone.
Asked about the arrest in Syria, Taoiseach Leo Varadkar said: "The information we have is that an Irish passport-holder, an Irish citizen, has been taken into custody, has been held in Syria.
"We don't know the details of that so I can't comment on it in any detail but what I can say is that any Irish citizen around the world is entitled to consular assistance and will get that."