LONDON – Soon after three young children and a woman in her 30s were wounded in a knife attack outside a Dublin school on Nov 23, rumours about the perpetrator’s nationality began to proliferate online.

The Garda Siochana, the Irish police force, did not reveal anything about the background of the man who had been tackled to the ground by bystanders and taken into custody after the attack.

But unconfirmed reports that he was an Algerian migrant began circulating in nationalist and far-right groups, according to disinformation researchers.

Alongside those: A call to gather in central Dublin, in what anti-immigrant voices framed as a stand against crime and in defence of Irish children.

What started as an online rumour spread to rioters attacking police, setting police cars on fire and looting stores in the worst unrest to hit Ireland in decades, the authorities said on Nov 24.

Some demonstrators carried banners reading “Irish Lives Matter”. Others vandalised hotels and hostels thought to be housing migrants.

Several police officers were injured, one seriously, and 34 people were arrested, Mr Drew Harris, the police commissioner, told reporters on Nov 24.

“We have not seen a public disorder situation like this before,” he said, adding that there was “an element of radicalisation” to the disorder.

A group of people had taken “a thimbleful of facts” and added “a bathful of assumptions – hateful assumptions”, he said.

Videos of the scene on the morning of Nov 24 showed a strong police presence and city workers scraping burnt debris off the roads and towing away damaged cars.

Irish officials condemned the rioters and called the disorder disgraceful, blaming a far-right faction for fuelling tensions by spreading misinformation about the knife attack.

Prime Minister Leo Varadkar said that police would fight back against “waves of ignorance and criminality”.

“Those involved have brought shame on Dublin, brought shame on Ireland and brought shame on their families and themselves,” Mr Varadkar said.

“This is not who we are. This is not who we want to be, and this is not who we will ever be.”

Researchers specialising in the spread of online extremism said the riots were an example of far-right groups capitalising on the discontent and disenfranchisement of some Irish people, at a time when many have struggled to keep up with the cost of living and a housing crisis.

Like many parts of Europe, Ireland has received an influx of newcomers in recent years as conflict, economic pressure and climate change have driven migration.

In the year leading up to April, the number of immigrants to Ireland reached a 16-year high of 141,600, according to official data, including more than 40,000 Ukrainians.

Dublin City University’s Professor Jane Suiter, who studies disinformation, said news of the knife attack on Nov 23 quickly spread through far-right groups, websites and social media spaces.

“It snowballed,” Prof Suiter said. “The protest was small in the beginning. But then young men started arriving from everywhere and converging.

“We need a lot more research and to be taking online radicalisation and extremism more seriously in Ireland.”

The disorder on Nov 23 followed a stabbing attack in central Dublin, in which a teacher and three young children were wounded outside a school at about 1.30pm. One of the children, a five-year-old, remained hospitalised in critical condition on Nov 24.

A man in his 50s, also injured in the attack, was taken into custody, police said on Nov 23, adding that the motive for the incident was unclear and that they were keeping “an open mind” at this stage of the investigation.

Within hours, a “riotous mob” had gathered at the crime scene and tried to break the police cordon, police commissioner Harris said.

The disorder escalated after 5pm, he said, and police deployed more resources.

Mr Harris said that the force would review its tactics on policing public disorder.

But he pushed back at assertions that officers had failed to contain the unrest, adding that they could not have anticipated its intensity.

Mr Varadkar said that Ireland would “modernise” its laws on incitement to hatred, and in the coming weeks would pass new legislation allowing police to better use closed-circuit television footage they collected on Nov 23.

Irish Justice Minister Helen McEntee told national broadcaster Raidio Teilifis Eireann that anyone charged with assaulting a police officer could face up to 12 years in prison. NYTIMES