Such links are still relatively rare, reflecting in part the scale of the actual problem, the lack of information on the issue, and the ingrained culture of Irish intelligence services to provide limited information.

Alexandr Ruzmatovich Bekmirzaev is originally from Belarus, and moved to Ireland around the year 2000, and is thought to have become a naturalised citizen in 2010.

It emerged on Sunday night that the 45-year-old was one of five people arrested by Kurdish militia in eastern Syria, who said the five men were IS fighters.

Bekmirzaev worked in various jobs in Dublin and left the city in 2013.

Garda sources said their understanding at the time was that he was going to fight in Syria.

His wife and their Irish-born child are understood to have left afterwards.

It has been reported that Bekmirzaev was facilitated by a man described in the High Court as Islamic State’s “foremost recruiter” in Ireland. He was deported last year.

The case of Bekmirzaev highlights the twin issues of Irish citizens who travelled to Syria and Iraq and the risks posed by those who might return — a process expected to accelerate given the significant military successes against IS in the region.

Estimates going back a number of years — and cited by the New-York based security organisation, The Soufan Grup — have said that in the region of 30 Irish citizens had travelled to Syria and Iraq over the last five or six years.

While the number is small, the ratio per head of population (seven per million) is higher than Spain (three per million) and similar to Germany (nine per million), though significantly lower than France (26 per million), Belgium (42 per million) and Sweden (31 per million).

Garda sources and sources in the Muslim community have pointed out that many of these joined military factions fighting Syrian President Bashar al-Assad or IS.

Gardaí have said their policy is to engage with those they know about as they go out and come back, but said this was based on the intelligence available.

What has happened to the 30 people is unknown to a significant extent, though five are thought to have died.

This includes Dubliner and convert Khalid Kelly, who was reportedly blown up by Iraqi forces as he was attempting a suicide bombing in November 2016.

While authorities don’t know for sure, he is thought to be the first Irish person to die fighting with IS.

Garda sources have said they don’t know the outcome of the others, including how many are still out there and fighting. It is not clear as to how many have travelled back to Ireland.

One report published in September 2017 said there was only a “small number of returned foreign fighters” which it estimated to be in the “low double digits”.

The report, compiled by the Financial Action Task Force, said Irish authorities indicated these people were under surveillance and subject to close monitoring.

It said there were only a small number of individuals in Ireland involved in international terrorism activities.

It noted the deportation in 2017, and a second one still going through the courts, but added: “However, other disruptive methods such as revoking the passports of persons who may be suspected of terrorist-related activities have not been actively utilised.”

Garda sources have pointed out that it was not possible to keep every person of interest under surveillance, which is a massively labour intensive endeavour.

“The amount of attention would depend on how active they are and are they associating with main players,” said one source.

“People go in and out of that. It’s not an exact science, you can’t grade them as such. It’s all based on intelligence.”

Sources said that Bekmirzaev was of “significant interest”, but that doesn’t necessarily mean he was constantly monitored.

One source said that “all the investigations and intelligence” to date showed that jihadist suspects in Ireland were involved in providing the likes of “false documentation or financing for terrorist groups abroad or recruitment to fight abroad, not physical attacks here”.

Last July, in the first such conviction, Irishman Hassan Bal was sentenced to two and a half years for fundraising for IS.

In June 2017, one of the three men behind the London Bridge terror attack, Rachid Redouane, had previously lived in Ireland. A Garda review established Redouane had no links to terrorism while in Ireland. A similar review will now be now conducted in relation to Bekmirzaev.

Regarding returning fighters, gardaí and EU agencies have expressed concern, given the military experience and radicalised views of the people concerned.

“There is certainly a fear,” said a garda source.

“It’s a fear right across Europe, for all police and security services.”

Figures show that three people were arrested for suspected jihadist activities in Ireland in 2017, compared to one in 2016.

The EU police agency Europol has documented a rise in jihadist attacks, accompanied by a decrease in their sophistication, citing the use of vehicles and knives in many incidents.

The Department of Justice has noted the change in the rhetoric of IS towards ‘personal jihad’ and self-motivated attacks in Europe.

The Government’s National Risk Assessment 2018 also referenced Europol’s view that a rise in returned fighters would “likely strengthen domestic jihadist movements” and magnify the threat to the European Union.

In an interview last May, Assistant Commissioner Michael O’Sullivan, head of Garda Security and Intelligence, said a “small number” of IS sympathisers were being monitored here.

He said lone wolf attacks were “probably the greatest concern” for all police and security services throughout Europe, adding: “It is a concern for us.”

The security chief said that of the estimated 30 Irish citizens gone to Syria and Iraq, a small number were still alive. He said “quite a number were deceased” and that some were missing.

He said: “Of concern to us certainly would be families returning and that is a concern for a lot of jurisdictions. These may have been families not involved directly in the war but whose children, particularly young males, may have witnessed some of the most appalling crimes known to man.”

He added: “They will return and they will become a challenge for a multi-agency effort — not just An Garda Síochána but other agencies as well. If and when they return, how they are managed from a welfare, education [point of view], so that is probably more of a concern at the moment than returning fighters.”

Ireland’s intelligence structures are due to be revamped under the Policing Commission report, with a new overarching intelligence assessment unit, called the Strategic Threat Analysis Centre.

It will bring intelligence from the gardaí, the Defence Forces, and other bodies together and will be headed by a new national security co-ordinator.

The body, based in the Department of the Taoiseach, will develop strategies and devise priorities for the individual agencies, which will retain operational independence.