The two most powerful ministers in the Lynch government in 1970, Charlie Haughey and Neil Blaney, were first sacked, then arrested and accused of conspiring to import guns. These were to go North to arm defence committees controlled by the Provisional IRA.
Also accused of attempted gun-running were Capt James Kelly, an Army Intelligence officer who had bought the guns in Germany; his interpreter, Albert Luykx, a Belgian businessman living in Sutton; and John Kelly, a Belfast member of the IRA.
The country went into shock that May of 1970 as rumours swirled. As the Dail went into a non-stop 36-hour debate, the Army's loyalty was said to be in question, with over 100 officers ready to demand a stronger policy on Northern Ireland. There were supposed to be plots by Saor Eire, a splinter IRA group, to kidnap or kill the Taoiseach, Jack Lynch. Neil Blaney was said to be planning a coup in Dublin and an invasion of the North.
Kevin Boland, who resigned from the cabinet in sympathy with Haughey and Blaney, accused the Taoiseach of "treachery" and using "Gestapo tactics" to tap the phones of politicians. Later the country would hear how the guns, when cleared through customs by Haughey, were to be taken by Capt Kelly to a Dublin convent, then to a monastery near the Border for later smuggling into the North.
Peter Berry, the elderly secretary of the Department of Justice, who aborted the gun-running plot by ringing Dublin Airport with Special Branch men, would tell how he took a vital phone call from Haughey while he was in the nude after coming out of his sauna.
Bonfires burned for Blaney when he returned to Donegal after charges against him were dropped in a District Court in July, but he needed an RUC escort as he drove through Northern Ireland.
Hundreds of Haughey supporters sang A Nation Once Again outside the Four Courts Hotel after the acquittal of all the defendants in October. Calling himself a patriot, Haughey demanded Lynch's resignation but soon afterwards voted for a motion of confidence in his leader.
So how did all this madness begin?
A year earlier on October 5th, 1969, less than two months after the Bogside in Derry and nationalist areas in Belfast had been under attack by the RUC and loyalist mobs, a meeting was held in a hotel in Bailieboro, Co Cavan. Ostensibly it was for the citizens' defence committees, set up to defend nationalist areas in the North, to pass on their needs for guns and training to the Dublin government.
The meeting had been arranged by Capt Kelly, who had been given the job of liaison with the defence committees following a series of decisions by the Lynch government in the aftermath of the August 1969 violence.
These decisions included improving intelligence-gathering in the North, giving £100,000 for relief of distress at the sole discretion of Haughey as minister for finance, and the setting up of a government sub-committee run by Haughey and Blaney to deal with the situation.
At the Bailieboro meeting were members of the IRA northern command, who were being offered assistance of various kinds from some members of the Lynch government. Out of the meeting came demands for guns for defensive purposes to be imported through the South.
Capt Kelly reported all this to his superior, Col Michael Hefferon, Director of Army Intelligence. The expenses for the meeting had come from Haughey's fund, and one of its aims was to split off the Northern IRA from its Marxist leadership in Dublin under its chief-of-staff, Cathal Goulding.
But the Garda Special Branch had its spies at the meeting and it made a separate report for Peter Berry, noting with concern that an Army officer was meeting with "subversives". The minister for justice, Micheal O Mor ain, passed on Berry's report to a cabinet meeting, but no attempt was made to stop Kelly's contacts with the committees, which were in Dublin regularly asking Lynch and other ministers for guns to defend their areas.
The Bailieboro meeting has been described by Capt Kelly as "the genesis of the Arms Trial". Out of it came a fatal confusion of aims.
Kelly, who was reporting regularly to Col Hefferon and to the Minister for Defence, Jim Gibbons, on his activities, believed he was carrying out government policy in trying to organise a supply of arms for the defence committees/IRA to protect their communities against eventual pogroms.
Blaney encouraged Kelly to get arms for the committees, especially after the IRA split in December 1969 into Provisionals and Officials. The latter were seen as pursuing a "purer" republican agenda of ending partition, while the Marxist Officials were suspected of trying to turn Ireland into a European Cuba.
Supplying guns for Northern nationalists was not viewed the same way in autumn 1969, while rows of Catholic houses in Belfast were still smouldering, as it would be today. Few in the South would have been prepared to stand idly by if pogroms against Catholics in the North had broken out and there had been widespread killing.
"If the midnight knock comes to our door, would not the gentlest of us love the feeling of security which a pike in the thatch can give?" asked the nationalist leader, Eddie McAteer.
The government itself issued a directive in February 1970 to the Defence Forces to prepare for a possible "Doomsday situation" in the North. The Army was to be ready to move across the Border and make weapons and gas masks available to nationalists if necessary.
With the British troops then keeping the peace in the North, the reason for such a directive is not clear. It may have been as a result of Blaney warnings that a new loyalist onslaught was being planned.
The upshot was that the government had a plan for the Army to move guns across the Border to assist nationalists if needs be, and Capt Kelly was working to buy guns for the same purpose. Kelly believed he was fully covered by his superiors in Army Intelligence and that he had political coverage from Haughey, Blaney and Gibbons.
Where did the Taoiseach stand in all this? He claimed the first he heard about the Kelly operation to fly weapons into Dublin Airport was on April 20th, the day before they were to arrive.
But the original plan was to ship the weapons, paid for out of the Haughey distress fund, from Antwerp to Dublin Port on March 25th, and Haughey had used his power as minister to allow them in without Customs clearance. This plan was foiled, possibly by British Intelligence, and Kelly then sent the guns to Vienna, to be flown to Dublin where John Kelly of the Belfast IRA would collect them.
Berry was alerted through Aer Lingus that the guns were coming and ordered the Special Branch to seize them. "There's a ring of steel around Dublin Airport," he told his minister, O Morain, who joked: "You'd think we were in f---ing Casablanca or somewhere". Haughey who had agreed Customs clearance was told by his personal secretary, Tony Fagan, the guns would be seized. Haughey then phoned Berry, who was in his sauna.
Berry's version of the call was that Haughey said the consignment would go directly to the North and the seizure would be "a bad decision". Haughey then said: "I'd better have it called off."
At his trial Haughey denied this account and insisted he believed the consignment was for the use of Military Intelligence, in accordance with the government's contingency plans, and he did not know whether it consisted of guns or not. The judge, Seamus Henchy, told the jury either Haughey or Berry was lying, and either Haughey or Gibbons was committing perjury over their differing accounts of another meeting.
The fate of Haughey and his co-defendants hung on whether the jury believed Gibbons knew in advance about Kelly's plan to import guns. If he did know and did nothing to stop it, then it was a legal operation as Gibbons was the minister authorised to import guns.
Col Hefferon, who was a prosecution witness in the first trial (it collapsed when the judge, Andreas O'Keeffe, withdrew), said in his statement to the Special Branch: "It is my opinion that Mr Gibbons knew that Capt Kelly was involved in assisting the defence committees in the North to procure arms".
However, it was revealed on Prime Time this week that this part of his statement was deleted in the Department of Justice, almost certainly by Peter Berry, before being given to the State prosecution team as part of the Book of Evidence.
The role of Berry's new minister, Des O'Malley, in this decision has now become controversial. This tampering with a vital witness's evidence to distance Gibbons from the Kelly operation did not have the desired effect.
After hearing Hefferon's evidence in court that he had briefed Gibbons on Kelly's plan to import arms for possible use by the defence committees, the jury took only 50 minutes to find Haughey and the three other defendants not guilty.
By then the Provisional IRA was arranging money and weapons from sympathisers in the US. Far from being a tool for the anti-partition policy of politicians in Dublin, it soon grew into a ruthless war machine, and deaths mounted.
But the Arms Trial removed Haughey and Blaney from positions of power and ensured that the Lynch Northern policy of unity only by consent of the unionist majority would never again be challenged, even when Haughey returned to power in 1979.
The Arms Trial by Justin O'Brien was one of the books consulted for this article