One of the undercover officers for whom Scotland Yard apologised for tricking women into sexual relationships carried out surveillance in Northern Ireland, say environmental campaigners.
Seven women were compensated for the deception, by five officers from two undercover units over a 25-year period.
Campaigner Kim Bryan said one of the officers spent some time in Northern Ireland and the Republic of Ireland.
She said she went to Belfast in 2005 with Mark Kennedy at his suggestion.
"Mark Kennedy organised the travel - he paid for the trip and we went in his vehicle so he drove us around Ireland when he came to visit," she said.
"We did three events in total when we were in Belfast.
"I don't know exactly what he was doing, but I suspect he used the opportunity to spy on activists in Ireland - in Belfast and Dublin and in County Clare where we were.
"I suspect that information was fed back to his superiors, but again, I think what's highly problematic is that we would not have gone to Ireland if Mark Kennedy had not paid for us to go to Ireland."
Last month, the Met made an "unreserved apology" and agreed financial compensation packages for seven women deceived into relationships with undercover officers who were spying on campaign groups.
They took place over 25 years until the units were disbanded in 2008 and 2011.
The secret unit that Mark Kennedy worked for was called the National Public Order Intelligence Unit.
Asked about its activities, the PSNI said the unit "did not operate in Northern Ireland".
A spokesperson for the Met said, when asked if the unit worked in Northern Ireland, "we neither confirm nor deny".
Jason Kirkpatrick, a US environmental activist now living in Berlin, said he also accompanied Mark Kennedy to Belfast where they ran anti-globalisation events.
He said one was at a bar in University Street and another was at a church in the Holyland area that offered its premises at the time to various groups involved in community work.
"We had a public education event at a bar called the Menagerie where we did a kind of a pub quiz on globalisation," he said.
"We played a kind of anti-globalisation bingo in a church in the centre - the City Church.
"Mark Kennedy was definitely at the City Church.
"We have photos and we don't understand to this day why this was such a big event that the British police would send an undercover spy to target us."
Her Majesty's Inspectorate of Constabularies investigated the use of the undercover police officers in 2012.
When asked for a comment about whether the undercover officers operated in Northern Ireland, it referred the BBC to a footnote in one of its reports that said: "The unit's remit covered England and Wales. They also worked with forces in Scotland and Northern Ireland."