A four-day royal trip has been hailed by both neighbouring states as a landmark in normalising relations following the hard-won peace in British-ruled Northern Ireland.

But it has also highlighted how dissident republicans still pose a threat, after Irish police found a "viable explosive device" on a bus in Maynooth, near Dublin, hours before the queen's arrival.

Police were tipped off by an anonymous caller and around 30 passengers were reportedly evacuated from the bus, which was heading for Dublin. The device was defused by the army, officials said.

There were also two false alarms as troops carried out a controlled explosion on a suspicious package found on Dublin's light railway system on Tuesday and investigated an apparent hoax device in the city on Monday.

The incidents came after dissident paramilitaries made a coded bomb threat in central London on Monday, the first of its kind outside Northern Ireland for 10 years.
Britain's Foreign Office and Buckingham Palace both insisted there was no change to the visit, the first to Ireland by a British monarch since the country won independence in 1922.

The last sovereign to visit Ireland was George V, the queen's grandfather, in 1911.

Irish Prime Minister Enda Kenny said there was no chance the visit would be cancelled despite the discovery of the bomb and said a "comprehensive security operation" was in place.

"This is an historic and symbolic visit and it is dealing with the conclusion of the past and a message for the future," he said.

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 there in April.

The dissidents remain a small minority 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.

British Prime Minister David Cameron, who is accompanying the queen, said the visit showed the strong links between the two countries, especially during the economic crisis.
"I believe Her Majesty's visit will be the start of something big," he said.

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 Cameron to release files about the incident.

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 George V stayed there.

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 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.

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.

"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," he told AFP.