The queen laid a wreath during a solemn ceremony at Dublin's Garden of Remembrance [EPA]

Queen Elizabeth II has begun the first visit to Ireland by a UK monarch in more than a century, despite a bomb being found on a bus near Dublin, the Irish capital.

The four-day trip is intended to highlight good Anglo-Irish relations following years of animosity and peace in Northern Ireland.

The queen visited the Garden of Remembrance, a central Dublin memorial that honours people who fought for Irish independence from Britain.

Al Jazeera's Andrew Simmons, reporting from Dublin, said: "That is a major act of symbolism, a potent moment."

The queen is the first British monarch to visit Ireland since its independence from the UK in 1921.

The cost of the security operation to protect the queen during her visit has been estimated at $42m.

'Viable' explosive

Earlier on Tuesday, defence forces said they had carried out a controlled explosion on a "viable" explosive device found on a bus heading for Dublin.

A member of the Irish police force stands near a security sealed telephone box in Dublin [Reuters]

"It was on a bus and by the time our team was called in the bus was evacuated and parked at a bus stop," an army spokesman said.

"The device was made safe in situ," he added.

Simmons said: "The [first] device was on the outskirts in the commuter belt. It was inside a luggage compartment of a bus, and that bus was headed for the capital."

"[There are] are no claims of responsibility but you can certainly say that it's highly likely that dissident Republican groups are involved in the attempts at disruption of this historic visit."

A controlled explosion was also carried out on a second device, which turned out to be a hoax, the defence forces said.

A third device found in the Summerhill area of Dublin also turned out to be a hoax.

'Major act of symbolism'

Enda Kenny, the Irish prime minister, said the queen's visit would not be affected by the incidents, and the British foreign office and Buckingham Palace, the monarch's official residence, had said the state visit would go ahead.

"This is an historic and symbolic visit and it is dealing with the conclusion of the past and a message for the future," Kenny told RTE state radio.

"These things happen when global personalities visit any countries ... and whether it be Ireland or other countries, adequate security arrangements are put in place."

Queen Elizabeth II's itinerary includes a visit to Croke Park, a Gaelic sports stadium where British troops killed 14 people in 1920 during Ireland's uprising against British rule.

Her arrival also coincides with the 37th anniversary of bombings in Dublin and Monaghan, the single bloodiest day in a three-decade sectarian battle over Northern Ireland.

Thousands of police shut down key roads in the Irish capital and erected pedestrian barricades for several kilometers, while 1,000 Irish troops are on standby during the historical visit.

Tuesday's events come a day after British police said they had received a bomb threat for central London from Irish dissident republicans.