Garda HQ is also reviewing security strategies in the wake of both the Nice truck massacre and the German train axe attack.

The revelations come as the head of Europol urged Ireland to modernise its interception laws to help tackle the threat of terrorism.

Rob Wainwright said that there is a “vacuum in the law” when it comes to Facebook, WhatsApp, and other social media which needs to be addressed.

Speaking at the MacGill Summer School in Glenties, Mr Wainwright said: “I think we haven’t yet reached a stage where we have the right, balanced, proportional, modern system of managing the way terrorists make use of the internet.

“I think there is a bit of a vacuum in the law and a vacuum in how we deal with that in society. As these countries move more and more to Ireland as Ireland becomes a data hub, it’s important Ireland gets that right in terms of how these different parts of our society work together maybe in a more constructive way.”

It is understood specialist gardaí are monitoring the public online activity of known suspects here to gauge their reaction to the recent terrorist attacks.

“Thousands of posts are looked at every week, and that is done all the time,” said one source. “If there is anything of interest, it is followed up.”

Speaking after the Nice attack, in which Mohamed Lahouaiej-Bouhlel killed 84 people, including 10 children, Tánaiste Frances Fitzgerald said that any public event here attracting hundreds of people would require “security of a different sort now”.

Mohamed Lahouaiej-Bouhlel

Gardaí are conducting these reviews, but security sources said it was “very, very difficult” to prevent someone using the likes of a truck from killing people in crowded public areas or someone, like the teenage refugee in Germany, from stabbing train passengers.

Officials pointed out that they were alive to the possible use of a truck in a terror attack because of statements by IS spokesmen in 2014 urging people to use rocks, knives, and cars to kill.

Senior officers are concerned at the reports from French government officials and German police that the attackers were not on their radar and appeared to be radicalised very quickly by IS-type propaganda, but were not apparently in any direct contact with them.

“Rapid radicalisation is an issue, but some people would know, like close family members,” said a security source.

“We would hope people would come forward beforehand. They have already done so in relation to foreign travel [sons intending to fly to Syria to fight].”

Gardaí yesterday conducted a terror attack training exercise at the Garda College in Templemore.

The threat here is assessed as moderate — possible, but not likely — the second of five stages.

It comes as a terrorism report by Europol, the EU police agency, found that the EU was facing “an elevated threat” from jihadists.