Leah looks at why the Queen hadn't visited the Republic of Ireland
The Queen has made history by becoming the first British monarch to visit the Republic of Ireland.
A massive security operation with thousands of police, one of the biggest the country has ever seen, is in place throughout the four-day trip.
She has been welcomed by President Mary McAleese, who invited the Queen to visit, at her official home in Dublin.
British and Irish leaders say they hope the visit will help continue to improve relations between the two countries.
The Queen laid a wreath at the Garden of Remembrance in Dublin, which was a big deal because it's dedicated to people who fought for Irish independence.
There have been some protests during the visit from people unhappy about the Queen's visit.
The army in Ireland found a bomb on a bus outside Dublin on Monday but destroyed it.
UK Prime Minister David Cameron will join the Queen and Duke of Edinburgh for part of their trip on Wednesday.
The Queen's visit is the first of its kind since the country's independence from the UK in 1922.
Thousands of police are involved in the security operation
King George V was the last reigning monarch to visit, in 1911, when what is now the Republic of Ireland was then part of the UK.
The relationship between the UK and Ireland has been one of the longest and most difficult in the history of Britain's monarchy.
Hundreds of years ago in 1171, Henry II invaded Ireland, and for centuries English kings and queens saw it as part of their lands.
In the mid-19th Century the potato crops in Ireland failed.
Much of the land there was owned by English landowners, a lot of whom did nothing to help - and the British government didn't intervene either.
This meant that about a million people in Ireland died, either from starvation or epidemic disease.
The monarch, as the main symbol of the British state, became one of the main targets of Irish anger.
By the early 1900s feelings of revolution against the Crown had built up.
The Queen was given flowers by Rachel, aged eight
This led to the Easter Rising in 1916 which was led by republicans - people who wanted Ireland to be completely separate from Britain.
Three years later the Irish War of independence began, a war between the Irish Republican Army and the British government.
In 1922 Ireland was split into the independent Irish Free State, whilst the United Kingdom kept six counties in the north - Northern Ireland.