The concern is part of a fear across the European Union, with reports from top EU terrorism officials of an increase in returning fighters in recent months, one that is expected to accelerate with the offensives against Mosul in Iraq and Raqqa in Syria.

“There is certainly a fear,” said one Irish security source, “it’s a fear right across Europe, for all police and security forces.”

The concerns follow the reported death last Friday week of well-known Irish radical Khalid Kelly, who was blown up by Iraqi forces as he was attempting to carry out a suicide bombing mission near Mosul.

His is the first known death of an Irish citizen in the region recent years and he is thought to be the first Irish suicide bomber for IS, who referred to him as Abu Usama al-Irlandi.

IS fan site claims (unconfirmed) this is the moment Khalid Kelly killed himself and several Iraqi militia.

I last saw him in Dublin in 2011 pic.twitter.com/CqlY3emQwT — Philip Boucher-Hayes (@boucherhayes) November 5, 2016

Security sources here said they were still trying to confirm whether or not Kelly, a 50-year-old convert to Islam, is actually dead.

“We are looking at that, but, given the nature of what’s happening out there, it is very, very difficult,” said one source.

As well as conducting checks abroad, they are trying to confirm his death through associates and family here.

Sources said that, given the profile of Kelly, they had been keeping track of his movements and confirmed he was in Ireland last March or April.

There were reports that, last March, he attended the deportation hearing here of a Jordanian man described by gardaí as IS’s “foremost recruiter” in Ireland.

Sources said they were examining the movements and communications of Kelly’s associates here, saying they were “quite a small group”.

Kelly was a “person of interest” for a long time for both the Garda’s Security and Intelligence Section and the Counter-Terrorism International section of the Special Detective Unit.

He was investigated and arrested on a number of occasions in relation to both suspected terrorism offences and for incitement to hatred.

Despite files being submitted to the Director of Public Prosecutions, he was not charged, let alone convicted, of a terrorism offence.

His death is thought to be the first known fatality of an Irish person in Syria or Iraq since four deaths in 2012 and 2013. Those deaths involved people in groups fighting the Assad dictatorship and were before the emergence of IS.

“There could be other deaths we don’t know about,” said the security source.

“There might be some people missing out there for one reason or another, they could be dead or alive.”

It is not clear if Kelly is the first Irishman to die with IS.

The investigation comes at a time when European authorities and security experts have predicted the return of foreign fighters from Syria and Iraq given the assaults on IS territory.

The director of EU’s police agency Europol said last month that flows of returning fighters had “slightly increased” in recent months.

Rob Wainwright said they were “not yet in high numbers” but added that “maybe Mosul and Raqqa will change that”.

More than 5,000 EU citizens have travelled there since 2011, including around 30 Irish people. The Irish figure includes those involved in humanitarian work in the region.