Queen Elizabeth II embarks on Tuesday on the first visit to Ireland by a British monarch since 1922 after troops defused a bomb near Dublin following threats by republican hardliners.

The landmark four-day trip is aimed at normalising relations between the two neighbouring states. But a visit intended to underline the progress made following the hard-won peace in British-ruled Northern Ireland also highlighted how dissident republicans still pose a threat.

Irish police said a 'viable explosive device' was found on a bus overnight in Maynooth, near Dublin, adding that they had been tipped off by an anonymous scare.

The device was defused by the army, police and the military said.

It came a day after dissident paramilitaries made a coded bomb threat in central London on Monday, the first of its kind outside Northern Ireland for 10 years.

Police sealed off roads near the sovereign's Buckingham Palace residence and carried out a controlled explosion.

Opposition to the queen's visit persists among a violent hardcore of republicans, who want British-ruled Northern Ireland to become part of the Republic, and a Catholic policeman was murdered in April.

They remain a small minority, with the major republican groups having joined the political mainstream, and officials are doing their best to ensure the 85-year-old queen and her husband Prince Philip get a warm welcome.

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

Doctor Patrick Geoghegan, a history lecturer at Trinity College Dublin, said 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 told AFP.

"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."

The royals will fly into Casement Aerodrome, southwest of the capital. The base is named after Roger Casement, an Irish nationalist executed for treason by the British in 1916.

The queen's arrival coincides with the 37th anniversary of the Dublin and Monaghan bombings by the Ulster Volunteer Force, a loyalist Protestant 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 British Prime Minister David Cameron to release files about the incident they say were withheld from an Irish judge who probed the bombings, amid claims of British collusion.

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

The Aras dates back to 1751 and used to house the viceroys who oversaw British rule in Ireland. Queen Victoria and king George V stayed there.

George V, Queen Elizabeth's grandfather, was the last British monarch to visit, exactly 100 years ago.

Following talks, the queen and the president head straight for one of the most sensitive moments of the trip -- a visit to the Garden of Remembrance, dedicated to "all those who gave their lives in the cause of Irish freedom".

Both McAleese and Queen Elizabeth will lay wreaths and the national anthems of both states will be played. Republican demonstrators will be kept far from the scene.

The couple's final engagement on Tuesday will be to visit Trinity College, one of Europe's finest universities, where they will view the Book of Kells, a ninth century gospel manuscript.