The Queen will make history today by becoming the first British monarch to visit the Republic of Ireland.
There have already been protests as not everyone in Ireland is happy about the trip.
But the British and Irish governments say it will improve the relationship between the two countries.
A massive security operation involving thousands of police is under way in Dublin ahead of the Queen's arrival later today.
The army in Ireland say they have made safe a bomb found on a bus outside Dublin this morning.
Why hasn't a British monarch visited for so long?
Thousands of police are involved in the security operation
The Queen's visit is the first of its kind since the country's independence from the UK in 1922.
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.
Irish parents make their feelings known using their baby's pushchair
In the mid 19th Century the potato crops in Ireland failed.
Much of the land there was owned by English landowners, most of whom ignored the situation.
This meant that about a million people in Ireland starved to death.
The monarch, as the biggest symbol of the British state, became one of the main targets of Irish anger.
Irish War of independence
By the early 1900s feelings of revolution against the Crown had built up.
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.
Leaders hopeful for the future
Despite this troubled history, leaders in the two countries are hoping that the Queen's visit is a big step towards improving their relations.