Image copyright PAcemaker Image caption Peter Robinson, pictured after his first court appearance at Ballybay, County Monaghan, over the Clontibret affair

The Royal Ulster Constabulary tipped off police in the Republic of Ireland (gardai) about a loyalist incursion into Clontibret, County Monaghan in 1986, according to files just released.

The loyalists included the then deputy leader of the DUP, Peter Robinson.

A note to the British Ambassador to Dublin refers to about 150 loyalists, "some wearing paramilitary uniforms and carrying cudgels" invading Clontibret.

They daubed slogans on a garda station and injured two officers.

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 police 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."

The official noted that Mr Robinson 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 Londonderry 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" following a court appearance in the Republic of Ireland over the Clontibret incident.

The two men made a formal protest at the Foreign Office about the "totally inadequate protection" given to Mr Robinson when he appeared in court in Dundalk, County Louth.

Image copyright PAcemaker Image caption There was a riot in Dundalk as Mr Robinson appeared in court

Image copyright PAcemaker Image caption The DUP leadership complained they were given inadequate protection in Dundalk

What happened in Clontibret?

On 7 August 1986, Peter Robinson marched with a group of loyalists into the County Monaghan village of Clontibret, in protest against what he claimed were inadequate security measures along the Irish border following Margaret Thatcher's signing of the Anglo-Irish Agreement.

The agreement had given the Republic of Ireland a constitutional role in the affairs of Northern Ireland for the first time.

Mr Robinson was subsequently fined £15,000 for his role in the protest.

The then-party leader Ian Paisley told a BBC documentary, broadcast months before his death at the age of 88, that the protest "shouldn't have been done".

When asked if there was a feeling within his family then that Mr Robinson had been making a leadership challenge - Mr Paisley was out of the country at the time - he replied: "Everybody has a right to decide for themselves what their answer to that is."

"I think he (Mr Robinson) thought that there was going to be a tremendous uprising as a result of all that, and that didn't happen," he said.

However, Mr Robinson said Mr Paisley's account was "a failure of recollection".

"He had to leave to go to a funeral in the US and I stepped in as his deputy into the Clontibret arrangement," said Mr Robinson.