Queen Elizabeth II embarks Tuesday on the first visit by a British monarch to the Republic of Ireland since it won independence, surrounded by a massive security operation.
In a highly charged trip aimed at reconciliation following the peace established in British-ruled Northern Ireland, the queen will undertake an historic four-day state visit to the republic.
Irish security forces are on full alert for the visits of the 85-year-old monarch and US President Barack Obama in the space of a week.
A 10,000-strong force will be deployed in the country's biggest-ever security operation amid threats from dissident republican paramilitaries opposed to any official British presence on Irish soil.
Mainstream republicans Sinn Fein, the political wing of the now-defunct Irish Republican Army (IRA), are also hostile to the visit by the queen and her husband Prince Philip.
Sinn Fein leader Gerry Adams on Saturday called the trip "premature and insensitive".
However, the ground-breaking visit has been welcomed by most Irish people, with a recent survey finding 81 percent in support.
Irish Prime Minister Enda Kenny has described it as "symbolic of the end to years of division and the start of a brand new relationship".
Ireland gained independence from Britain in 1922, but no British monarch has visited since the queen's grandfather King George V in 1911.
The itinerary does not sidestep difficult issues, with Queen Elizabeth visiting some of the most sensitive sites in Anglo-Irish relations.
After being welcomed by President Mary McAleese, the pair will lay wreaths at the Garden of Remembrance in Dublin, dedicated to those who died fighting for Irish freedom.
On Wednesday, the queen visits Croke Park stadium, where in 1920 British forces shot dead 14 people in a reprisal attack. It was a key moment in the Irish independence struggle.
The programme also includes lighter moments, with visits to the Guinness brewery, the national stud and a traditional market.
However, despite the official bonhomie, the security forces are taking no chances.
Fears were heightened when a Catholic policeman was killed by a car bomb in Northern Ireland last month, in an attack blamed on dissident republicans opposed to the peace process.
The clampdown includes some 8,000 police and 2,000 soldiers, with many Dublin streets barricaded off.
Police on both sides of the Irish border have arrested several dissident republican suspects ahead of the visit. Some fear such groups will attempt to hijack the visit with an attack.
The Real IRA paramilitary splinter group said the queen was wanted for "war crimes" and was "not wanted on Irish soil", threatening to ensure that she and "her cheerleaders get that message".
And in a move that could unsettle republicans, brigadiers from the Ulster Defence Association -- loyalist paramilitaries on the other side of the fence -- have been invited to a wreath-laying ceremony commemorating the World War I dead.
A state visit would have been unthinkable during the Troubles, the three decades of sectarian strife between Northern Ireland's Catholics and Protestants.
The royal family was also caught up in the violence. Prince Philip's uncle Earl Mountbatten was assassinated by the IRA in 1979.
Ties between the neighbouring states improved markedly during peace talks in Northern Ireland, which led to the landmark 1998 Good Friday agreement, which largely ended the bloodshed.
Even so, Queen Elizabeth has visited 129 countries before setting foot in the Irish Republic.
The visit "demonstrates the closeness of our relationship with our nearest neighbour, and how far this has progressed over the last decade", Britain's Foreign Office said.
Britain is Ireland's largest trading partner, while Britain exports more to Ireland than China, India, Brazil and Russia combined.
London has contributed seven billion pounds ($11.3 billion, eight billion euros) to the bailout package for Ireland's collapsed economy.
British Prime Minister David Cameron will also make his first official trip to Ireland, joining the state dinner on Wednesday along with Foreign Secretary William Hague, when attention will focus on the tone of the queen's speech.
The visit comes with the British monarchy on a high, following the wedding of Prince William and Catherine Middleton.
Three days after the queen leaves, Obama will visit Ireland and head to the village from where one of his ancestors emigrated in the 1800s.