Irish army chiefs feared the British and nationalists would call for Irish troops to be sent over the border in response to near civil war in Northern Ireland, state papers have revealed.
While the idea would never have got past the cabinet table, a secret aide memoire compiled for ministers on November 7, 1985 - the week before the Anglo Irish Agreement was signed - warned of the scenarios.
In documents released from the Taoiseach's office under the 30-year rule in the Republic, advisers said the more likely reaction to the agreement would be harsh and confrontational.
They contemplated anything up to "intercommunal fighting".
But the idea of a cross-border mission was rejected by defence chiefs because they said the army's size and equipment was "hopelessly inadequate".
They also thought an assignment into Northern Ireland would not have a clear military objective while Irish soldiers could only serve overseas with the United Nations.
The memoire warned: "Depending on its scale and intensity could lead to pressure for intervention by the defence forces either by request from beleaguered nationalist communities or by request from the British Government if it thought its forces were unable to cope or if it wished to involve the Republic for political reasons (prelude to withdrawal or share the responsibility for what would by then be seen as a failed initiative).
"Depending on the scale of the fighting there could also be political pressure in the Republic for intervention on the basis of 'we cannot stand idly by'."
The defence minister at the time, Paddy Cooney, had got the advice after summoning the Council of Defence - made up of the Chief of Staff, Adjutant General, Quarter-Master General, a junior minister and his department secretary - on August 28, 1985. He rejected any possibility of Irish soldiers going north without a reform of the law.
Mr Cooney said deployment would "carry with it the near certainty of fatal casualties".
And he also warned that reforming laws governing the defence forces to let them operate in Northern Ireland would "certainly raise apprehensions" on both sides of the border.
Mr Cooney also hit out at Fianna Fail: "It is sadly ironical, that the agreement designed to advance peace in NI should now, following the hostile reaction by Mr CJ Haughey, find itself commanding only minority support on the island."