The undercover policeman Mark Kennedy was in the vanguard of militant anti-capitalist protesters who attacked Irish police officers at an EU summit in Dublin marking the accession of eastern European states to the union, Irish anarchists have told the Guardian.

Protesters who took part in actions with him allege that his involvement went further than just observing. They allege that he also made visits to Dublin to help train protesters and encouraged other activists to attack the police. This raises further questions about his role as an undercover officer and backs up suggestions he acted as an agent provocateur.

The latest revelations have prompted one leftwing member of the Irish parliament to brand the UK police's secret operation akin to the illegal activities of British state agents operating on the island during the Northern Ireland Troubles.

Michael D Higgins of the Irish Labour party is demanding that the Irish government seek an explanation from Britain as to why one of its undercover policemen was operating in the republic.

"This kind of activity is totally unacceptable … There are many of us who are familiar with the destructive consequences, in terms of democracy, that have flowed from this kind of activity from the 1970s on in relation to Northern Ireland," the veteran Labour TD said.

Anti-capitalist activists operated with Kennedy, while they believed him to be a fellow protester called Mark Stone, during violent clashes between a small band of demonstrators and the Garda Siochána's riot squad on May Day 2004.

At the time the area around Dublin's Phoenix Park – the largest park in any European city – was sealed off by a huge security cordon as police sought to protect the heads of EU states who were in Dublin for the ceremony at the Irish president's residence.

One Northern Ireland-born anarchist, who asked not to be identified, said Kennedy had stayed with him in Dublin in the days before the demonstration.

Although the vast bulk of the demonstrators were peaceful, about 500 attacked the gardaí. The focal point for the violence was the gates leading to the Phoenix Park on the Navan road. At one stage the garda deployed water cannon to drive back a spearhead of masked demonstrators.

One of those taking part in the protest recalls being astonished by Kennedy's action on the day. "I saw him taking off his balaclava as he was coming out of the crowd. I was amazed that someone would stand close to police lines and take his mask off."

Before the protest it was clear the police had intelligence on the activities of some anti-capitalist activists who had travelled across the Irish Sea from Britain, suggesting that they had an informant in the protest movement. A unit from the garda's Pearse Street station smashed its way into a flat on Leeson Street in Dublin occupied by English anarchists and arrested several people.

The Irish anarchist who played host to Kennedy said that in the build-up to May Day 2004 the Englishman was in a militant mood. "Some people arrested from the UK were kept here for two months but he did not get nicked for that. He was one of the people who were encouraging a confrontation with the garda up at the Parkgate," the activist said.

He said Kennedy made at least two other visits to Ireland over the next two years. These included acting as a trainer on a programme for anarchist activists later in 2004 on civil disobedience. Two years later he joined the mass demonstration against the visit of George Bush to Ireland.

"I found him very personable, very affable. I never suspected him. Growing up in the north of Ireland you know there are people out there who are informers but I never had any strong doubts about him," said the anarchist.