by Paul Reynolds
BBC News Online world affairs correspondent
Bombs were hidden in "French peas" cans
German saboteurs claimed they were planning to attack wartime Britain using exploding cans of processed peas, according to secret files.
The MI5 documents show that three men who landed on the southern coast of Ireland in 1940 were found with four bombs hidden inside cans labelled "French peas".
The saboteurs claimed they were for use against Buckingham Palace.
But an MI5 assessment discounted this claim, saying the explosives were "of the most primitive kind".
"This seems rather fantastic", a report released by the National Archives on Friday stated.
"It is perhaps possible that all three were anxious to convey to the Eire authorities that their desires were against the belligerent British..."
MI5 reckoned the bombs might have been "travellers' samples" to show the IRA the kind of devices which could be used to help a German invasion of Northern Ireland.
The three agents were landed by dinghy near Cork, but their exploits were shortlived.
Their tactic, of asking the first person they met if they could be taken to the IRA, did not work.
The man took them to the police instead.
The plot was dismissed as amateurish by MI5.
"It illustrates the lack of organisation with which the Abwehr (German Intelligence) operated at times," it concluded.
The files show a close relationship between the Irish and British authorites, despite Irish neutrality.
MI5 knew of the arrests and saw transcripts of the interrogations almost immediately.
Other files give details of another German-IRA plot, codenamed "Plan Kathleen" - a proposal for the IRA to take part in an invasion of Northern Ireland.
This plot also got nowhere.
Northern Ireland as seen in 'Plan Kathleen'
It was discovered when Irish police raided a house in Dublin in May 1940 and arrested an IRA leader named Stephen Carroll Held.
They found a used parachute, wireless sets, large sums of US dollars and documents about Plan Kathleen.
In the documents were instructions for spying on naval bases in Northern Ireland and on the whereabouts and movement of troops.
There was also a plan, complete with coding instructions, to set up a transmitting station in the north to communicate directly with Berlin.
The Irish raid took place on 24 May. MI5 must have been told at once, because its own report on the case is dated 27 May.
There was no immediate sign of whoever had used the parachute, but 18 months later a German agent named Hermann Goertz was arrested and it turned out to have been him.
His Irish interrogators reported: "The general idea of the plan was an invasion of Northern Ireland by German forces with the assistance of the IRA who would come in from Ballyshannon and Dundalk.
"It was thought that the British would be forced to break Irish neutrality to deal with the threat at Carlingford Lough.
"This, it was considered, would have the effect of turning the people of the Twenty Six Counties against the British."
The plan had originally been thought up by an Irish civil servant from Belfast named Liam Gaynor.
There was in fact no person called 'Kathleen."
And in the event, there was no invasion.