In the Star Trek: The Next Generation episode "The High Ground," Data mentioned that Ireland had been reunified in 2024. That's the only official reference to the event that I can think of, so everything else (including the actual date) was made up by me. It is a holiday celebrated in Ireland (in the future), and by people of Irish descent around the galaxy, including my primary Trek fanfic character, Jax DeSabel.
Here is an excerpt from a scene in one of my posts in Imperial Entanglements (which sparked the whole idea of writing the history of the Mirror Universe)
"...To celebrate the day in 2024 when the Irish Republican Empire finally conquered the last remaining opposition to its total dominion of the Earth, its oldest enemy and closest neighbor, England. Just two years earlier, the Empire had finally taken the last two members of the once-glorious United Realm. Ah, once upon a time, the U.R. had itself attempted to take over the world, and came very close indeed. And then the IRE began its long war of conquest. After Wales and those traitorous Scottish dogs fell, all that stood in our way was England, and on March 4, 2024, we took control of that as well. And the IRE held the Earth for hundreds of years. Once they had the planet, they started spreading out into space, conquering more worlds, to form the Terran Empire."
"Um, okay," said Jax. "On my Earth, March 4, 2024 was the day Ireland reunified. By that time, the United Kingdom wasn't so big, it was just made up basically of England, Wales, Scotland, and Northern Ireland, which had long ago been stolen. But we finally got it back, and Ireland was whole."
"You call that a reason to celebrate?" asked M-Jax. "That's not winning, that's breaking even."
"Whatever, let's just shut up and drink."