The Pagan Federation are also in Ireland, although I don't know what they organise. Worth checking their website though and maybe have a look for Irish pagan/witch groups on Facebook or similar, a lot of local events can be posted in those sorts of groups but wouldn't necessarily show up in a Google search.
I believe Newgrange also live-stream the winter solstice sunrise as Newgrange is aligned to it like Stonehenge is. That could definitely be fun to watch.
If you like the King Arthur idea, you could also read some of the Gaelic tales that are connected to the winter solstice as it's such a great time for cosy reading and storytelling. Some believe that the Celtic love God Aengus was born at the winter solstice because his father the Dagda made the sun stand still for days to conceal Aengus' birth, and Aengus later lives at Newgrange, which is aligned to the solstice. So, you could read the story of Aengus' birth or his taking of Newgrange from his father or foster-father by trickery!
A Scottish tale, also involving Aengus, relates to this time of year, so you might also want to read it. The winter crone Goddess the Cailleach kidnaps Brigid, who rules over springtime, and keeps her hidden in a cave so that winter can continue. Aengus dreams of this and is determined to rescue Brigid, but he can't cross the sea to get to her in the winter storms. So, he cleverly borrows 3 days from August so he can cross the sea in gentle, sunlit waters 😂The ending varies - some say that Brigid is then able to defeat the Cailleach to bring in the spring, and others believe that the Cailleach becomes Brigid (i.e. the Cailleach is the old, fierce form of the gentle spring maiden Brigid who then grows and matures into a mother in the summer, then a middle-aged woman in autumn and finally becomes the Cailleach again each winter).
They're beautiful stories so worth a read at any time of year, but the solstice is the perfect time! Many modern pagans who follow the Wheel of the Year then honour Brigid at Imbolc (also called St Brigid's Day) on the 1st February, so I recommend looking into the Wheel of the Year if it interests you - there are 8 seasonal holidays consisting of the 2 solstices, 2 equinoxes and then the 4 Celtic fire festivals. I love feeling so connected to the seasonal changes, of course the Wheel of the Year is a religious thing for me as a pagan but I also find it so beneficial for my mental health. Modern life is so disconnected from natural cycles and rhythms, and this really helps to remedy that, in my experience.
Sorry for the ramble! You've had some lovely ideas on this thread 