Suggestions for European trip between Christmas and New Years

Cold? Did you ever look on a climate table? It lists not one minus-value. If you look for warm in December you shall look for southern hemisphere with summer.

What are your interests?
What do you want to experience?
From where do you travel?

For such a short trip - where can you easily fly into on a direct flight? It would be helpful if you put your home town in your profile.

Days are shorter anywhere in northern Europe in December and colder. Ireland will be wetter than many countries too. The time between Christmas and New Year certainly isn't off season and it's a time when there is limited or no public transport on certain days, such as no trains 26 December. There are plenty things to do indoors, but many breweries will be closed over this period. Dublin has fewer big sights than say London, Rome, Paris or Berlin.

I would head to Berlin which I find to be better value than Dublin and there's more to see. It won't be warm...

For warm -- we used to visit Hawaii at Christmas.
Mele Kalikimaka!
As noted by others, Christmas to New Years is high season in many places.

If you want Europe, it will be cool. Rome is at about the same latitude as New York, so while some places are cooler, none of it is warm. So embrace the weather and look for a place that has something going on. Naturally, Budapest is my first choice, but there is a lot to see do and enjoy in the winter in much of Europe.

Thank you for the advice you have all given so far!

To answer a few questions...
We are flying out of Orlando (there is a direct flight to Dublin from there)

We usually like to spend half of our days doing tourist attractions (museums, castles, etc) and the other half relaxing (reading in a local coffee shop, picnics, pubs, etc)

I checked the climate table and feel comfortable with the projected weather. We had a great trip to Philly last December and it was 40s and rainy most of the time.

At that time of year I’d choose a city break with lots of indoor attractions and a good amount of charming coffee shops, pubs, bars, etc. Ireland wouldn’t appeal to me in the winter since so much of its charm is in exploring the countryside and the days will be super short (as in, sun rises at 8am and sets at 3:45pm) and worse than cold (in my opinion) possibly quite wet. Not great conditions for extended time outdoors.

Instead I’d suggest Berlin (already mentioned), Vienna, or Amsterdam. All three places have tons of interesting museums and other indoor attractions along with cosy cafés, interesting cocktail bars, and good restaurants. In Vienna at least, a couple of the Christmas markets may still be open (not sure about Berlin).

The Christmas markers will still be open in Budapest. Also, if you hit an Orthodox country, with Christmas being on January 7th you will still be able to enjoy Christmas. Lviv comes to mind.

Why Europe? You're going to spend 2 days just traveling there. I think you could do better either flying to Seattle (cool and rainy but rarely snow, lots's of museums, tourist stuff, outdoor activities, and craft beers), or Quebec (European culture with Canadian flair).

If you must do Europe I'd look seriously at Lisbon, Portugal.

When we don’t go on ski holidays, my go to has been London. I love seeing a different play every night and enjoy the museums during the day. Your love of pubs will be quite satiated. 😉

If you go to Spain, the whole time is part of their 12 days of Christmas. It starts on December 25 but their big day is January 6 (also known as Three Kinds Day and/or the Epiphany). In Southern Spain, the weather should be pretty comfortable and the days reasonably long.

You actually will have 4 days on the ground, so to speak. I would be off to Paris in a flash. Four days in Paris off season? What could be better.

As Brad has mentioned, Southern Spain will be quite pleasant. Much of Andlucia will likely be warm during the day, temperatures much better suited to exploring on foot and the Day of Kings on 6th January is more celebrated than Christmas Day. We were in Mallorca between Christmas and over New Year last and we had great weather however with the exception of Palma a lot of the island's restaurants were closed but this is less likely in the big cities of the mainland such as Malaga, Seville etc.

In Berlin one of the nicest Christmas markets at Gendarmenmarkt will still be open until Dec 31st.

For what you are planning to do Berlin is a real good option, also with one of the biggest European NYE parties at Brandenburg Gate. Tip: Do that photo first before stages will be built up :-)

We have 4 UNESCO world culture heritage palaces in Potsdam - just a short regional train ride.

Some more tips for NYE in Berlin, also some local traditions such as eating a Berliner but Berliners eat Pfannkuchen but all eat the same.