Luggage storage at Cordoba

I read somewhere about luggage storage at the bus station in Cordoba gets full quickly.

Yes. That was my experience, but it was a few years ago - maybe it's changed? In any case, we came in on the high speed train from Madrid. We made a point to be the first off the train, we raced (ran) through the station, out, across the street, down (what seemed at the time) the long block to the bus station. Ran inside. Among the many coin lockers there, we found just a few that were empty. My wife grabbed one and held it for us, while I went down to a counter that was selling small items, bought a pack of gum or a bottle of water to generate coin change (tip: arrive with a pocket full of coins ready to avoid this step). Walked back to our locker, paid. As I was completing the transaction, I saw several other (slower) travelers show up from the same train, who were disappointed to find all the lockers were now full. So, if nothing's changed, it may be kind of a race to get a locker.

That said, I would not let this deter me from including at least a few hours stop in Cordoba. The Mezquita is one of the most spectacular places in Europe, well worth the trouble of racing for a locker. The rest of Cordoba is cool, too, but even if all you have is 3-4 hours, it's worth the stop.

Which would mean taking the early train and spending about 5 hours in
Cordoba before we would to be in Malaga for our 7:00 pm flight to
Dublin.

That would be a really, really bad plan, IMHO. Never plan on traveling far on your departure day to reach your flight out. Always spend the last night in (or very near) the city where your outbound flight goes from. There are just too many ways that something can go wrong causing you to miss your flight (traffic jam, train strike, construction, accident, equipment issues, personal or local emergency, etc.). Unless you're prepared to shell out really really big $$$ for another ticket home if you miss your flight.

This is really a long shot, but we stayed at the Hotel Mezquita . It's just steps to the Mezquita. I'm wondering if hotels ever hold luggage for day trip travelers? You might email them and ask them.

I also would caution against being to far from your airport on the day of your flight. I have found trains very reliable in Europe, but you never know. I will add that we had booked a train Cordoba to Madrid and then to Toledo. When we got to the Cordoba train station our train was cancelled! To make matters worse, the next train was full. So we were delayed in Cordoba more than an hour. (and needed to rebook our train to Toledo because we would have had only 5 minutes to get thru the huge train station)

Looks like I am going to be running to get one of those lockers on December 29

I would also agree that Cordoba merits more than a day trip. We really liked the charming town and spent a lot of time just wandering in the Jewish quarter. Yes, the mezquita is an absolute must see, but the rest of the area is pretty nice, too. We liked the town of Cordoba more than Granada.

We took the train from Seville to Madrid and stopped in Cordoba for the day. We had no problem finding some lockers to store our carryons while we visited the Mezquite. This was in September 2017.

Don't go to Cordoba between Granada and Seville. Take the bus between Granada and Seville, then visit Cordoba as a day trip from Seville.

You may find, however, that three days in Seville is hardly enough.

Looks like I am going to be running to get one of those lockers on December 29

It's not too late to start training. Get out there and hit the track! ;)

If my memory is right (that's usually a 50-50 proposition), when you come up to street level from the train platform, the shops and main entrance are on your right. The bus station is across the street on your left, just cross the street and you are there. I don't remember if the lockers are on ground level or one level down. I do remember that the toilets there were the pits - OTOH, the toilets in the train station are as clean as any in Spain.