A coded bomb threat in London by the Real IRA has sparked fears that a similar warning here will create a security nightmare for this week's visit by Queen Elizabeth.
The caller issued a recognised codeword used in the past by the dissident republican gang and it led to a five-hour shutdown near Buckingham Palace yesterday morning.
Senior garda officers said last night the alert in London underlined the need for the strict measures being implemented here to protect the queen during her four-day visit.
Gardai have no intelligence indicating that the Real IRA has an active terrorist cell operating in London but have passed on details to Scotland Yard of known sympathisers living there.
Some of the suspects are related to veteran republicans that have featured in Real IRA activities here in the past five years.
Scotland Yard believes the threat call was made from the Republic but had not pinned down the location last night.
Last month a masked member from the Real IRA declared its opposition to the visit at a rally in Derry and threatened to kill more police officers. It was responsible for the murder of two soldiers at Massereene Barracks in Antrim in March 2009.
The security alert in London was based mainly around The Mall, although the call, which was made on Sunday night, did not specify a time or a location.
It was the first coded warning by dissident republicans in London for more than a decade.
Now garda anti-terrorist officers are concerned that a similar attempt will be made today in a bid to create chaos during the queen's visit.
The queen is due to arrive at Baldonnel airbase at midday and around two hours later will be taken to the Garden of Remembrance, where the city centre streets have been shut down by gardai since Saturday and protest groups will be kept back from the secured site.
Activists from socialist republican group eirigi have been forced to move back from Parnell Square, which was flagged as a potential flashpoint, and will stage their demonstration elsewhere in the capital.
In addition to the streets shut at the weekend, the remaining sides of Parnell Square and O'Connell Street in both directions will be closed from 7am, while at midday the closures will also extend to Westmoreland Street, D'Olier Street, College Street, Nassau Street and South Leinster Street.
After a short stay at the Garden of Remembrance, the queen will then visit Trinity College for about an hour and return to her residence for the three nights at Farmleigh in the Phoenix Park.
As a ban on private planes was put into operation, the Air Corps and Army surveillance teams prepared to scan the skies before the arrival of the royal flight, with the Naval flagship, the Eithne, patrolling Dublin Bay.
Gardai expect some minor disturbances during the visit and two special sittings of the district court will be held each day at Cloverhill, if required, until next Tuesday. Two judges have been assigned round the clock to the court and have been on stand-by since yesterday.
The London bomb warning, meanwhile, has resulted in a cross-border focus on the Real IRA, whose suspected chief of staff has already been arrested and questioned.
Three other key figures in the gang's leadership and their associates were last night under human and technical surveillance. One of them, who is regarded as the group's "commanding officer" in Dublin, is living in a south county suburb but is originally from Cork.
Another leading figure is based in Derry and operates almost an independent republican fiefdom in Derry while the other prime suspect is from the Dundalk area.
Justice Minister Alan Shatter said last night that the vast majority of people were looking forward to the queen's visit and those who engaged in terrorist stunts were just attention seekers.