Newly-released British documents from 1985 show that some members of the gardaí may have been passing more information to the British authorities than ministers were aware.
A secret report says the British Special Branch and MI5 enjoyed "excellent relations" with their garda opposite numbers, with more co-operation than was suspected by some Irish ministers
In mid-1985, as the negotiations on the Anglo-Irish Agreement continued, British Prime Minister Margaret Thatcher sought an assessment of the level of security co-operation between the two countries.
The possibility of more security co-operation from Dublin was one of the main benefits Mrs Thatcher expected from any agreement.
The report, drawn up in May 1985 by Britain's security co-ordinator, said successive Irish governments had encouraged co-operation but were reluctant to admit this publicly, because they were sensitive to charges of collaboration.
And he said "we believe that even those Irish ministers who are aware of it in general terms may be unaware of its extent".
He said Britain's Special Branch and MI5 had "excellent relations" with garda intelligence and security branch, and "benefit from a degree of co-operation and from a flow of intelligence which we believe to be at a greater level than is suspected by at least some Irish ministers.
"A small number of garda officers ... are... prepared to be extremely helpful" - and their co-operation made "a major contribution to combating the present terrorist campaign on the mainland".