Lockdown for Queen Elizabeth landmark Ireland visit

The historic four-day trip, the first by a British monarch to the Republic of Ireland since it gained independence from London in 1922, is a landmark moment aimed at normalising relations between the two neighbouring states.

However, a visit intended to underline the progress made following the hard-won peace in British-ruled Northern Ireland is taking place surrounded by the highest security.

Central Dublin was in a police clampdown amid Ireland`s biggest-ever security operation, while dissident republicans opposed to the peace process and the British sovereign`s visit made a coded bomb threat in London on Monday.

Police sealed off roads near the sovereign`s Buckingham Palace residence and carried out a controlled explosion after the first coded warning outside Northern Ireland for 10 years.

Such warnings have traditionally been used by Irish republican paramilitaries before attacks.

Dissidents opposed to the peace process have been resurgent in Northern Ireland in recent months, murdering a Catholic policeman in a car bombing in April.

Opposition to the queen`s visit persists among hardcore and violent republicans, who want British-ruled Northern Ireland to become part of the Republic.

However, they are a small minority and officials are doing their best to ensure the 85-year-old queen and her husband Prince Philip receive a warm welcome.

Patrick Geoghegan, a history lecturer at Trinity College Dublin, said that inviting the queen was a statement of Ireland`s confidence in both its independence and its relationship with Britain.

"We`ve had a long relationship; sometimes it`s been very close, sometimes it has been acrimonious, but over the past number of years, we have had a very warm friendship," he said.

"They are our closest trading partner, they are our neighbours who helped us out during the recent IMF (International Monetary Fund) bailout, and we rely so much, for trade and for tourism, on the United Kingdom."

A 10,000-strong force is being deployed at an estimated cost of 30 million euros ($42 million), with reports saying even the navy will be deployed off the Dublin coast to prevent against a possible missile strike from the sea.

The royals will fly into Casement Aerodrome, southwest of the capital.

The former Baldonnel military airbase was renamed in 1965 after Roger Casement, an Irish nationalist executed for treason by the British in 1916.

The authorities spent Monday combing the sides of the road outside the airbase with police stationed along the perimeter.

The queen`s arrival coincides with the 37th anniversary of the Dublin and Monaghan bombings, perpetrated by the Ulster Volunteer Force, a loyalist Protestant paramilitary group. Some 34 people were killed, making May 17, 1974 the deadliest day of the three decades of strife known as the Troubles.

In an open letter to the queen, survivors and victims` families have pressed for British Prime Minister David Cameron to release files about the incident they claim were withheld from an Irish judge who investigated the bombings, amid allegations of British collusion.

The royal couple`s first port of call is the Aras an Uachtarain, President Mary McAleese`s official residence, for a ceremonial welcome.