The Queen bowed her head and laid a wreath at Dublin's Garden of Remembrance today in a hugely symbolic and historic act of reconciliation between Britain and the Irish Republic.
The national anthem was also played as the monarch stood beside President Mary McAleese at the site where Ireland commemorates the men and women who died resisting British rule.
The first official royal visit since independence took place amid the biggest security operation ever mounted in the state with thousands of police and armed soldiers on the streets of the capital.
A crowd of dissident republicans, opposed to the peace process in Northern Ireland, were held back by riot squads not far away but, apart from a few arrests and missile throwing, trouble did not disrupt an extraordinary occasion.
Students and staff cheered and applauded at Trinity College as the royal party arrived for a private viewing of the Book of Kells.
The 15 minute wreath-laying ceremony at the garden was the first major engagement in a carefully planned four day trip, which will tomorrow see Ireland's war dead remembered at Islandbridge, and Croke Park where 14 civilians were shot dead by British troops in 1920.
Former Irish prime minister Brian Cowen, who formally invited the Queen last year, declared relations between Britain and Ireland have been transformed by the peace process.
"The vast, vast majority of the people of Ireland wanted to see this day come," he said.
The Garden of Remembrance opened in Easter 1966 to mark the 50th anniversary of the 1916 Rising when seven signatories to Ireland's Proclamation of Independence, backed by the 1,000 strong Irish Citizen Army, launched a revolution against British rule beginning with the takeover of the GPO a few hundred yards away on O'Connell Street.
It is dedicated to "the memory of all those who gave their lives in the cause of Irish freedom".
The Queen's attendance at the garden is a required element of the State visit under diplomatic protocol.
Scores of streets in the capital remained sealed off as the Queen and Duke of Edinburgh travelled from the President's residence in the Phoenix Park to the garden and Trinity.
Despite groups of well-wishers on some routes the royal couple did not meet the public at any stage and it is unlikely they will at all this week.
The Queen touched down at Baldonnel aerodrome around midday wearing green in a symbolic gesture to Ireland and its people.
She changed into an ivory coat with olive green trim for the day's later engagements.
The royal visit began on the anniversary of the 1974 Dublin-Monagahan bombings when 34 men, women and children - including an unborn baby - were killed in no-warning car bombs in the greatest loss of life in a single day of the Troubles.
Campaigners have demanded Britain open secret intelligence files on the atrocities, accusing security forces of colluding with loyalist paramilitaries.
Former prime minister Bertie Ahern, who played a key role in the Northern Ireland peace process, attended the garden with Mr Cowen as members of Ireland's Council of State.
"I think anyone who understands Irish history will understand what we have just seen," Mr Ahern said.
The Queen will spend the night in Ireland's official state residence, Farmleigh in the Phoenix Park, the former home of the Guinness brewing dynasty.
Tourism and business chiefs are attempting to tap into the huge interest around the historic trip and these efforts are to be further cemented by a British-Irish chamber, the first time such close and formal business links have been established.
During the visit to Trinity the Queen met former Irish president and human rights campaigner Mary Robinson whom she invited for tea in 1993.
Dr Robinson said President McAleese had played an invaluable role in securing the visit - the first to Irish shores by a British monarch since King George IV in 1911 and the first in the history of the Irish Republic.
"It is wonderful that now it has come to fruition for both islands," Dr Robinson said.
"It would be good if she could meet larger numbers of people during her visit but security has to be taken seriously and there were sufficient indications that there might be a problem."
Dr Robinson said the visit would not have been possible without the peace process.
"For our relations between these two countries this is a very good visit," she added.
The Queen's engagements begin tomorrow with a visit to the Guinness Storehouse and a "windows" tour of Dublin from the Gravity Bar hosted by soon to be BBC broadcaster Ryan Tubridy.
The monarch will also meet Taoiseach Enda Kenny at Government Buildings before attending the war memorial at Islandbridge, Croke Park and a state dinner at Dublin Castle.
The Queen will make her only public address at the banquet which the President will also address.