Queen Elizabeth and Prince Philip (R) view a portrait of Queen Elizabeth I at Trinity College Dublin May 17, 2011.  [Photo/Agencies]

DUBLIN  - British Queen Elizabeth II on Tuesday began a four-day state visit to Ireland, becoming the first British monarch to set foot in the republic.

It is the first visit since George V in 1911 visited Ireland, which was still under British Rule.

The British Queen and her husband, the Duke of Edinburgh, were greeted by Ireland's Deputy Prime Minister Eamon Gilmore. Wearing emerald green, the British Queen was given a guard of honor by members of the Irish Air Corps at Casement Aerodrome, a military airbase in south Dublin, before making her way to Aras an Uacthtarain, the residence of Irish President Mary McAleese.

It was the invitation of McAleese and the success of the 1998 Good Friday Peace Agreement that has led to the Queen's historic visit. At the presidential office, the Queen observed a formal military salute by the Irish defence forces and signed a visitors' book before planting a tree in the garden.

Ireland is undergoing the largest security operation in the history of the country with up to ten thousand policemen and security forces working to ensure the Queen's safety during her trip.

The recent killing of a police officer in Britain's Northern Ireland and an increase in the overall threat posed by dissident paramilitaries has led to the massive security presence.

On Monday evening, Irish police made safe what they are describing as a viable explosive device on a bus destined for Dublin.

Irish Prime Minister Enda Kenny was also present at the presidential residence. Prior to her visit, Kenny said the Queen's visit "marked the excellent relationship that has developed between the two states based on mutual respect, partnership and friendship."

The visit of the Queen is being welcomed by the majority of the population but some argue that the visit is premature given that the six counties of Northern Ireland remain part of the United Kingdom.

During her four-day stay in Ireland, the Queen will visit several sites of historic resonance and will deliver a speech in Dublin Castle. The itinerary includes a wreath laying ceremony at Dublin's Garden of Remembrance.

A visit to Trinity College, Ireland's oldest university. And a highly sensitive visit to Croke Park, the home of Gaelic sports in Ireland and the site of the massacre of fourteen Irish people by British troops in 1920.

The Queen will also indulge her love of horses with a visit to Ireland's National Stud and will lastly travel south to visit an English style market in south Ireland's Cork city before returning to Britain on Friday.