Short answer: yes, if you have the kit and the mindset. Longer answer: it's complicated.
Water temperatures off Ireland in December through February sit around 7–9°C. That's cold — not arctic, but cold enough that anything other than a properly fitting drysuit will end your dive early. Most local Irish divers have been through the drysuit transition and won't touch a wetsuit for winter diving. If you're visiting from somewhere warmer, take a breath and factor in the time and cost to get certified or at least get some pool time in a drysuit before committing to a winter sea dive in Ireland.
The flip side: fewer people, better visibility on settled days, and some sites that simply come into their own when the summer crowds have gone. Killary Harbour in Connemara, for example, is sheltered enough to dive in winter when the Atlantic-coast sites further west are blown out. Dive clubs from around the country converge on Killary through the winter months for exactly this reason.
The plankton bloom that can reduce visibility in early spring actually brings marine life — more food means more activity. You might see things in January that you'd miss in August simply because the animals are hungrier and more visible. And the seals at Dalkey Island or around the Connemara coast are present year-round, often more curious in the quieter winter months.
That said, winter diving in Ireland requires a serious approach to weather. Storms can wipe out the entire west coast for weeks. Plan B isn't optional — it's essential. And it needs to be a proper plan, not just "we'll head to a different site." You need to know which sites have sheltered approaches, which require a boat, and which are completely exposed to Atlantic swells coming from the north or west.
The Irish dive season generally runs March to October for a reason. Outside those months, you're playing the odds with a higher skill floor. If you're qualified, experienced, have proper kit, and approach it with the respect it deserves — winter diving in Ireland is underrated.