Make any sense to do a day in Dublin in January?

It keeps changing ... I think I'd have to cut out Cordoba and Granada, so it would basically be something like eight nights spread across Madrid, Valencia, and Seville.

Dublin is worth visiting for a full day, or indeed for longer. January isn't the ideal time, as you must know. But whether this Dublin stopover is "worth it" really depends on how you evaluate the choices you have about flights. You're asking people their opinions without explaining what those choices are. I don't know how to help you with this.

Maybe it's as simple as noting that two days in Spain is better than one day in Dublin, especially in January, because it's twice as long and the weather will be better. But do you really need someone else to tell you this?

@Dick no, the things you point out are the reasons I'm asking whether it would be really fun to have a day in Dublin. The flights are substantially improved, and my son has been dying to see Ireland for reasons that he is unable to specify, so I'm just trying to see how this clocks for people. The weather is a downside as we will have to bring coats that maybe we otherwise wouldn't bring, but I don't think it is a dealkiller.

Yes! I was there in January with my 12yoa son, and we enjoyed it. No crowds.

You don't need a big coat. It can certainly be cold (and feel colder due to the damp), but layers are always the right answer. Packable rain jacket, puffer (which we stored in a small pillow case when not in use), whatever long-sleeve you're using for sun protection in Spain. Try a Buff or similar neck gaiter/hat thing. Packs up small. Warm socks. You'll be fine. Duck into a pub as needed (kids are generally allowed before 8pm).

And you probably already know this, but you're likely to save even more money by booking separate tickets. Fly over and back to Dublin, and purchase VERY cheap tickets between Dublin and Spain. Use Skycanner to find those low fares. We flew from Dublin to Tenerife, and I don't think we paid $100 for two one-way tickets (and we had 22" rolling carry-ons). Lots of Irish people vacation in Spain in the winter.

@G3rrycee I am planning this, because it will be an award ticket back from Dublin. Unfortunately, I did already use Skyscanner and the tickets to Dublin are not particularly cheap, nor are there direct flights from the cities I'd prefer to fly out of. It looks like I will have to get back to Madrid to get the flight, which will cost money and time. Kind of a bummer.

Does Sky scanner include Ryanair?

As well as Madrid they fly at least to Barcelona and Malaga, maybe other places in Spain as well.

I don't know what you call cheap, but they should be some of the best fares going.