It was hardly an ‘invasion’ in the true military sense. Nothing like the 200,00 Allied forces that invaded Iraq in 2003 or the German invasion of the Soviet Union in 1941 or of Poland in 1939. Yet Peter Robinson’s nocturnal excursion along with a group of 150 loyalists across the border into the quiet County Monaghan village of Clontibret on August 7 1986 was dubbed an ‘invasion’ by some sections of the media.

It was more like a sortie, a raid, an incursion or an infiltration. His intention was to show what he believed were the gaps in cross-border security, following the signing of the Anglo-Irish agreement in 1985. Yet it was the RUC who tipped off the Gardaí about his plans, according to Stormont papers recently released by the Public Record Office of Northern Ireland.

Historian Éamon Phoenix who has researched them says that a note from a Northern Ireland Office official from the Political Affairs Division to the British Ambassador to Dublin refers to about 150 loyalists, “some wearing paramilitary uniforms and carrying cudgels” entering Clontibret.

They daubed the slogan “Ulster is Awakening” on a Garda station and from what I myself remember of the day in question, on some walls including that of a Church of Ireland school. The crowd also injured two Gardaí.

The BBC reports that the note said: “The RUC’s action in tipping off the Gardai during the night of 6-7 August about the incursion by Peter Robinson and his loyalist thugs was also warmly appreciated in Dublin, according to Michael Lillis [of the Irish Department of Foreign Affairs].”

The NIO official who wrote the note told the ambassador: “We have done our little bit here by holding Irish hands in the [Anglo-Irish] Secretariat and feeding them with material for their hourly reports to their ministers during periods of particular tension.” The report notes that the crowd dispersed when gardai fired shots into the air.

“Robinson, who appears to have lingered behind deliberately, was arrested and held in custody for 32 hours (during which he refused all sustenance provided by the gardai, preferring the wholesome Ulster food brought to him by his wife) before being charged with four offences, including assaulting gardai and causing wilful damage.”

Although Mr Robinson was already in Ulster, this reference is to the breakfast brought to him by his wife Iris during his detention at Monaghan Garda station.

The official noted that Mr Robinson (who first appeared in court in Ballybay) was granted bail to appear in court in Dundalk on 14 August.

Other loyalist shows of strength planned to take place on the same night as Clontibret were limited by RUC activity to Swatragh in County Derry where a group of masked men, some carrying firearms, marched through the nationalist village, causing some damage to property. Both incidents were condemned by the British and Irish governments. For its part, the DUP hailed the operation “as a clear indication of the absence of cross-border security”.

A separate file reveals that Peter Robinson and his party leader, Ian Paisley, felt they “narrowly escaped with their lives” and made a formal protest to the British Foreign Office about inadequate protection, following a court appearance in Dundalk over the Clontibret incident.

Peter Robinson later took over from Ian Paisley as DUP leader and First Minister of Northern Ireland.