The IRA was able to tap Garda telephone calls, the Irish government accepted.
During a security summit in London in 1986, the British wanted to know about possible infiltration of police communications.
Then-Secretary of State Tom King said the "sophistication" of the Provos was "impressive" and asked Dublin's Justice Minister, Michael Noonan, about Garda precautions.
Mr Noonan responded that the force was switching to a new nationwide radio network for rural divisions and that a contract for something similar was in the pipeline for the Dublin Metropolitan Area.
However, an Irish official intervened to say "we could not commit ourselves at this stage to saying that, in the context of Garda/RUC communications, the new equipment would be secure and this was something that needed to be examined".
Notes of the top-level meeting, just released into the National Archives, said: "As regards telephone communications, it was accepted that at present, these were not secure from IRA penetration."
During the same summit, Mr King said recent arms finds by the Garda in Sligo and Roscommon had "cheered up" the RUC greatly.
He said it would be of enormous assistance if the southern force could now find the IRA's mortar factory, which was successfully being used to make bombs to target RUC stations and was "boosting their morale".