Two or three decades ago, the thought of a former Irish Republican Army man dining at the same table as Queen Elizabeth in Windsor Castle would have been impossible to fathom.

Martin McGuinness at Windsor Castle

Martin McGuinness, the deputy first minister of Northern Ireland, attended a state dinner at Windsor Castle on April 8, 2014. (Dan Kitwood/Associated Press)

But when Martin McGuinness was part of a state dinner at the Queen's residence outside London this month, it was a striking example of reconciliation from a monarch who turns 88 today, all the while remaining a significant diplomatic force.

"As I look at the whole reign, ever since 1952, I think that the best decade of her reign has been this one," says Ninian Mellamphy, a professor emeritus at Western University in London, Ont., and a longtime royal watcher.

"She's been astonishingly effective as a diplomat and as a statesperson. It's a great argument against possibly retirement because she's certainly managing her advanced years astonishingly well."

There have, of course, been signs she is slowing down.

International travel is essentially off her royal itinerary, although she did go to see Pope Francis at the Vatican a few weeks ago and is expected in France for events commemorating the 70th anniversary of D-Day in June.

But long-haul air travel seems to be behind her now, and first-in-line Prince Charles, who comes to Canada with his wife Camilla next month, is assuming more of his mother's Commonwealth and other duties.

Along with Charles, other members of the Royal Family have been doing Commonwealth visits. Charles's son Prince William, his wife Kate, the Duchess of Cambridge, and their nine-month-old son Prince George are Down Under right now, visiting Australia and New Zealand.

At home, however, the Queen's diplomacy was on full display during a state visit earlier this month by Michael Higgins, the president of the Republic of Ireland and the first Irish head of state officially welcomed to Britain since Irish independence almost a century ago.

A dinner hosted by the Queen and Prince Philip included Higgins and McGuinness, Northern Ireland's deputy first minister who was also an IRA commander when the infamous group murdered Lord Louis Mountbatten, Philip's favourite uncle, off the Irish coast in 1979.

"For the Royal Family, relations between the U.K. and Ireland are not simply a political matter, but a personal one as well," says Carolyn Harris, a Toronto-based royal historian and blogger.

Troubled past

There's no doubt the two countries have a difficult past, dating back hundreds of years. More recently, more than 3,600 people on both sides of the Northern Ireland conflict were killed since 1969. The Troubles, as they came to be called, cast a dark and painful shadow over life in the two countries for generations.

Harris says it's crucial that visits such the recent one by Higgins, and by the Queen to Ireland three years ago, are taking place "because there have been centuries of distrust and bad feeling and certainly in previous reigns, Ireland [was] treated as inferior."

queen mcguinness handshake

Queen Elizabeth greets Northern Ireland's deputy first minister, Martin McGuinness, during a reception at Windsor Castle on April 10, 2014, two days after a state dinner organized as part of Irish President Michael D. Higgins's state visit to Britain. (Luke MacGregor/Associated Press)

In a speech during the dinner at Windsor Castle, the Queen acknowledged the difficult relationship the countries have had, and the hope that now exists for a more peaceful future.

"The goal of modern British-Irish relations can be simply stated. It is that we, who inhabit these islands, should live together as neighbours and friends. Respectful of each other’s nationhood, sovereignty and traditions. Co-operating to our mutual benefit. At ease in each other's company," she said.

"After so much checkered history, the avoidable and regrettable pain of which is still felt by many of us, this goal is now within reach."

She also vowed that "my family and my government will stand alongside" Higgins and his ministers as they mark the upcoming anniversary of events that led to the creation of the Irish Free State nearly a century ago.

Special resonance

And considering what happened then, the armed and bloody insurrection by Irish republican forces aiming to end British rule in Ireland at the same time as Britain was involved in the First World War, her comments take on a special resonance.

"If you think of what went on from 1916 to December 1921, who could ever have said that they could even think of asking her in some way to participate in the commemoration of that?" says Mellamphy, who came to Canada from his native Republic of Ireland five decades ago.

He considers there has never been a time of "so much harmony" between the countries.

Royal wit

The Irish may be known for the wit, but Queen Elizabeth can provoke a chuckle at their expense, too.

During a state dinner at Windsor Castle, the monarch noted that it took someone "of Irish descent, Danny Boyle, to get me to jump from a helicopter."

Elizabeth, or at least a parachutist who was dressed remarkably like her, made a dramatic entrance into the opening ceremony of the 2012 London Olympics in a James Bond spoof directed by Boyle.

"This relationship between her and Higgins just as heads of state has been remarkable and the kind of rhetoric they have used has been amazingly positive and I find everything about it to be amazingly hopeful."

Mellamphy sees that diplomacy, plus the diplomacy on display when the Queen met Pope Francis at the Vatican, as significant.

"In her dealings with various people such as the Pope who is about 10 years younger than herself and the president of Ireland who is much younger still, she's been astonishingly diplomatic, genial and civil in quite an admirable way," he says.

"I've never imagined that she could have such a good 10 years in her life."

Those years come at a time in life when many others would be retired. And 2013 saw a number of European and Middle Eastern monarchs and an aging Pope Benedict step aside.

But all indications are that Elizabeth won't be following in their footsteps right now.

"The Queen's situation is different," says Harris.

"She was crowned in a religious coronation in 1953 rather than the secular installation ceremonies that take place in continental Europe and made a vow for life to serve as Queen, so I don't think that we will see Queen abdicate despite the trend towards abdication in continental Europe of the last year."

Harris says it will be interesting to see how the Queen's schedule plays out in the coming year, "as it certainly depends on her health." The visit to Rome earlier this month was rescheduled from last year because she had gastroenteritis.

The health of Prince Philip, who turns 93 in June, could also be a factor. He was out of the public eye for a few months in 2013 after abdominal surgery and has had other health scares in the past couple of years.

In the meantime, the Queen continues with an active schedule and shows no signs of abandoning some of the things that have been important to her throughout her life, including horses. She was pictured quite happily taking a springtime ride on a pony near Windsor Castle a few weeks ago.

Queen Maundy Thursday

Queen Elizabeth gives Maundy money to a pensioner during the Maundy Thursday service in Blackburn, England, on April 17, 2014, just days before her 88th birthday. (Jack Hill/Associated Press)

On Thursday, she handed out Maundy money to 88 men and 88 women — one for each year of her life — at the cathedral in the northwest England town of Blackburn.

"It’s important to remember the Queen Mother lived to be 101 and continued to have public engagements to the end of her life," says Harris.

"It's certainly possible that the Queen may continue to be an active public figure for another 10 years if she follows the example of the Queen Mother."