Donal Billings, aged 66, also made bomb threats relating to the State banquet for the British monarch at Dublin Castle and against Cork Airport, from where the Queen was departing the country.
The court heard that in one bomb threat, the caller said he was from the “Republican Brotherhood Squad”, that there were mortars set for Dublin Castle, adding “this is for the Queen”.
In another threat, the caller said there were two bombs in the toilets at Cork Airport on the day the Queen was due to fly out. In a series of threats over five days in May 2011, Billings told gardaí there was a bomb at the Sinn Féin headquarters in Dublin and on a second bus.
Garda investigations, including phone analysis and surveillance, resulted in the identification of the suspect, who was arrested on May 20 and found in possession of a SIM card and a mobile phone connected with making the death threats. Mr Justice Tony Hunt praised the gardaí saying it was a “well executed and comprehensive investigation”. The court heard because the first threat by Billings on May 16, regarding the bomb on board a Dublin-bound bus, resulted in the seizure of a functioning explosive device, gardaí treated his subsequent threats “very seriously”, although no bombs were located in those cases.
Billings was convicted at the Special Criminal Court last month of possessing an explosive device and four additional charges of making bomb threats during the royal visit.
The court heard that Billings, from Dublin but living in Drumlish, Co Longford, had a previous conviction in Northern Ireland from 1973 for unlawful possession of explosives and sentenced to eight years in prison.
Mr Justice Hunt said it was an “obvious feature of concern” that an explosive device was found on a bus with a large number of passengers, which could have caused “great injury and destruction” if detonated, either accidentally or not.
Detective Inspector Pat Finley, who led the investigation, said this fact, in the circumstances of a visit of a foreign dignitary to the country, meant the subsequent threats were “taken very seriously”.
A technical report on the bomb on the bus said there was no doubt the device, if initiated, could cause “a large amount of destruction” and that anyone in the vicinity of it “could have experienced serious injury”.
Sentencing will take place on December 15. Billings faces a maximum sentence of 14 years for the bomb charge.