Irish President Michael D. Higgins met Britain’s Queen Elizabeth on Tuesday on the first ever state visit to the U.K. by an Irish head of state. Higgins, 72, and his wife Sabina met with the Queen, 87, three years after her groundbreaking visit to Dublin, now widely considered a diplomatic triumph, heralding what some have called a golden age in the relationship between the two nations.

During the four-day state visit, which concludes on Friday, Higgins and his wife will be the Queen’s guests of honor at Windsor Castle, a privilege afforded to only the most honored visitors.

On Tuesday afternoon, Higgins gave the first address of an Irish head of state to the Houses of Parliament, and paid tribute to the relationship between Ireland and the U.K.

“I acknowledge and salute all those who have selflessly worked to build concord between our peoples. I celebrate our warm friendship and I look forward with confidence to a future in which that friendship can grow even more resolute and more productive,” Higgins said.

“The ties between us are now strong and resolute. Formidable flows of trade and investment across the Irish Sea confer mutual benefit on our two countries. In tourism, sport and culture, our people to people connections have never been as close or abundant.”

A state banquet hosted by the Queen was scheduled for Tuesday evening, as was a visit to Prime Minister David Cameron’s office on Downing Street on Wednesday.

In a striking example of the pace of political change in the relationship between the two nations, the North’s Deputy First Minister Martin McGuinness of Sinn Fein attended the state banquet hosted by the Queen, a development that would have been considered unthinkable just a few years ago. While Sinn Fein acknowledged that McGuinness’s presence at the banquet would be problematic for some Republicans, McGuinness himself praised the role of the Queen in healing the relationship between the two nations.

"I was tremendously impressed by the very solemn way that she commemorated those Irish Republicans who lost their lives in the struggle for independence, how she acknowledged the importance of the Irish language and, probably most important of all, when she acknowledged that she had wished that things had been done differently or not at all," McGuinness said of the Queen’s state visit to Ireland in 2012.

"That was very, very impressive and I think that it is quite clear that this is a woman that is playing a leadership role."

On Monday photographers captured the streets of Windsor, which were decked out in Irish Tricolors ahead of the president’s visit. After travelling to Windsor from London on Tuesday accompanied by Prince Charles and his wife Camilla, Higgins and his wife were met by the Queen and her husband Prince Phillip.

Speaking in Irish, Andrew Seddon, captain of the Queen's Company Grenadier Guards, invited Higgins to inspect the guard of honor. Higgins then gave a ceremonial red coat to the regiment’s mascot, an Irish wolfhound named Domhnall of Shantamon.

The centuries long conflict between the two countries were also acknowledged on Tuesday when Higgins visited Westminster Abbey to lay a wreath at the Grave of the Unknown Warrior. He also stopped at the Mountbatten Memorial which marks the death of Lord Mountbatten, a close cousin of the Queen who was killed in an IRA attack in 1979.

There are several events on the schedule before Higgins departs for Dublin on Friday, among them a Northern Irish-themed reception hosted by the Queen on Thursday at Windsor Castle, an Irish concert at Royal Albert Hall, and a banquet hosted by the Lord Mayor of the City of London.