Queen Elizabeth arrived in Ireland for an historic four-day state visit, the first by a British monarch since Irish independence, DPA reported.
The queen, who arrived to Dublin's Baldonnel military airport amid heavy security, was welcomed by the Deputy Prime Minister Eamon Gilmore and Britain's Ambassador to Ireland Julian King.
Dressed in emerald green, the queen was presented with a floral bouquet by an 8-year-old girl from a North Dublin school.
After walking through a guard of honour, she was brought by car directly to the president's official residence, Aras an Uachtarain, in Dublin's Phoenix Park for a ceremonial welcome.
She would then have lunch with President Mary McAleese and plant a tree to mark her visit.
Irish Prime Minister Enda Kenny said he hoped that the queen would have an "enjoyable visit and receive a fitting welcome in Ireland."
British Prime Minister David Cameron stressed the "genuine friendship" between Britain and Ireland in advance of the visit Tuesday.
He said there were "hard-headed business" interests between the two nations, citing that Britain did more trade with Ireland than with Brazil, Russia and India combined.
"There are some 6 million people with Irish grandparents in Britain with 3 million people travelling either way between the countries on an annual basis," Cameron said. "This is the start of something big."
Security jitters ahead of the historic visit were heightened by the discovery of a bomb on a Dublin-bound bus late Monday.
The bomb found in Maynooth, 60 kilometres south of Dublin, was made safe by army bomb disposal experts overnight.
There were several hoax bomb scares in Dublin and at courthouses in various parts of the country Tuesday morning.
Central Dublin, which is surrounded by a ring of steel, was reported to be "very quiet" Tuesday morning with many streets including the main thoroughfare O'Connell Street closed off.
Later in the afternoon, the queen was to visit the sensitive site of the Garden of Remembrance to lay a wreath at a memorial which commemorates those who died in pursuit of Irish freedom.
Ambassador King said the queen's engagements had been "designed to pay respect to the Irish state and to reflect on the future relationship between the two states."
Ireland gained its independence in 1922 after a campaign of armed resistance against British rule known as the War of Independence (1919-21).
The last British monarch to visit Ireland was George V in 1911. Relations between the two countries have been strained for much of the time since, then owing to the fallout from the conflict leading to independence and the bloody violence in Northern Ireland.
When an independent Irish state was established in 1922, the six largely Protestant counties of Northern Ireland remained British.
The Good Friday Agreement, which paved the way for power-sharing in Northern Ireland, was signed in 1998 after almost 30 years of violence between Catholics who wanted a united Ireland and Protestants who sought to maintain the link with Britain.
The agreement marked the end of most of the worst of the violence, although a small number of breakaway nationalists continue to advocate the use of force in pursuit of a united Ireland.
The greatest threat to security during the visit is seen as coming from the dissident group the Real Irish Republican Army, a designated terrorist organization in Britain, also banned in Ireland.
There was some disappointment that the queen was not expected to go on any walkabouts, but the Irish police have said she would be visible from certain vantage points during her engagements around the city.
Police said peaceful protests would be allowed, but no protesters will be permitted within range of the Royal party.