And that means they were also fighting the British Crown.
Just as it was when William's grandmother, the Queen, came here and bowed her head to the dead on her historic visit in 2011.
The garden, in a quiet corner of the Irish capital, honours the memories of the those fighter who died in six uprising across several generations.
William and Kate paused and laid a wreath at the garden on Tuesday.
Their message read: "May we never forget the lessons of history as we continue to build a brighter future together."
The British and Irish national anthems were played as well as The Last Post for the fallen.
It was a poignant moment in the bright March sunshine.
The garden was opened on Easter Monday in 1966, on the fiftieth anniversary of the Easter Rising and is dedicated to "the memory of all those who gave their lives in the cause of Irish freedom".
In 1916, a small band of Irish Volunteers had taken over Dublin’s General Post Office and other key buildings in a major revolt against British rule.
They took on the might of the British Empire and held out for more than a week, despite being vastly outnumbered.
Nearly 500 people were killed. The dead came from the rebels and from British Army, who had, by then, amassed in Dublin in their thousands.
But civilians were also killed.
The British response, along with the execution of 16 leaders of the rising, significantly turned Irish public opinion against Britain and the British Empire.
Ultimately, the Easter Rising led to the creation of the Irish Free State in 1922.
In the garden was Michael Chester whose grandfather fought the British in 1916.
He told us: "I was here when Her Majesty Queen Elizabeth came here", before adding "and its amazing that she came here and paid her respects to the people who lost their lives."
What about the visit by Prince William today I asked, a future British King?
"Time heals everything and I think it’s about time that we put our hand out", Mr Chester said.
He added his grandfather would be "delighted" to see members of the British royal family at the event.