Terror experts have warned the Irish-UK border makes us vulnerable to Barcelona-style attacks and that bollards should be placed in pedestrian zones to protect against Isil supporters.

Writing in today's Sunday Independent, Sir Ivor Roberts, a UK diplomat and Counter Extremism Project board member, warned Ireland is vulnerable to Isil planning an attack from here because of how easy it is to cross the border into Northern Ireland.

He believes Ireland could be used as a base for Isil to launch attacks in the UK before retreating here again to launch further attacks elsewhere in Europe. Brexit also poses security challenges as the Irish and UK Governments seek to keep the border as fluid as possible when Britain exits the EU.

His comments come as a Garda watch list monitoring jihadi sympathisers has doubled to more than 70 as networks here provide logistical and fundraising support for Islamist activists.

"Ireland's land border with the UK - and paradoxically both countries' determination to keep the border as transparent as possible - exposes Ireland to the prospect of jihadist cells planning an attack in the Republic, executing it in the UK and then retreating to the Republic to plan further attacks in the UK or further afield," writes Sir Ivor.

An Algerian man, aged in his 40s, was arrested in Dublin earlier this month by a Garda terror unit as part of an operation with London Metropolitan Police. Gardai have also carried out arrests in Waterford where a man was charged with funding international Islamic terrorism earlier this year.

Joshua Molloy, a former British Army soldier who fought against Isil with Kurdish militants in Syria, warned Irish high streets remain vulnerable to attacks similar to those previously carried out in Barcelona, Nice, Berlin and London. Mr Molloy, who is embedded within Isil-run social media channels, urged steps must be taken to prevent copycat attacks.

"Considering vehicle attacks are the modus operandi of a terror movement where an individual terrorist becomes the entire organisation's commander for a day, it might be prudent to place bollards on either end of our pedestrianised streets," he said. "There is a sense in Ireland that this is something that could happen, but probably won't. This would all change the moment a van crashes into pedestrians on a busy city centre street."

Both men have also written in the Sunday Independent about the role social media plays in such attacks.

Mr Molloy highlights how Telegram, an encrypted messaging app, was filled with chatter hailing the Barcelona attackers last Thursday. Isil supporters used emojis to discuss the van attack on Las Ramblas with an air of giddiness.

"An army of cyber mujahidin have carved out a virtual caliphate and carry out media jihad on a daily basis, spreading Isil propaganda, such as guides for carrying out vehicle attacks or how to make bombs," said Mr Molloy.

"It is on the internet where they truly remain and expand a vast network of supporters from across the globe."

Sir Ivor called for these platforms to be policed more efficiently.

"Isil has repeatedly used such platforms to propagate their poisonous message and through encrypted chats to trigger specific attacks, often amplifying massively the voices of Islamist clerics.

"Telegram's founder, Pavel Durov, rejects calls to take down private Islamic State, Al Qaeda and Taliban chats, claiming that 'the right for privacy is more important [to Telegram] than our fear of bad things happening, like terrorism. Of course we need to balance the right to legitimate privacy with the requirement to prevent avoidable bad things happening. But for terrorists to be able to use social media sites as safe havens for their murderous plans tips the balance in their favour too far."

Sunday Independent