After spending time in Ireland, then London, I'd like to visit Wales but only have 3 days(one is a traveling day). From London is taking a train to Northern Wales too far? I am really interested in history and castles so I thought I might go to Conwy area. Is there a better area for seeing castles and historical things? Would I be better off in southern Wales and Cardiff? Help!

North Wales is great for Castles, but if you want to get to see a good few of them, you probably need to hire a car.

Conwy, Caernarfon, Beaumaris, Harlech, Criccieth come readily to mind. They range from well-preserved (or restored) to characterful ruins. There is also a selection of smaller castles, mostly in poor condition but interesting to see.

Interesting old towns, too. And the bridges over the Menai Strait are remarkable. A number of manor houses can be visited.

And great mountain and coastal scenery, which you might as well look at while you are in the area!

Cardiff is probably easier but does not have as many top class castles. There is the one in the city itself with Castell Coch and Caerphilly very close. Chepstow is a little to the east.

What you would get would be a direct train service whereas London Euston to Llandudno has several changes apart from the mid-morning service which is direct at 3 hours and 17 minutes.
London Paddington to Cardiff Central is just a few minutes over 2 hours.

One thing that North Wales has got over the south East Wales is scenery. The Cardiff area is heavily industrial.

Near Cardiff besides the castles mentioned by PP are Caldecott (medieval banquet, book ahead), and Penhow, home of the Seymours (as in Jane, third wife of Henry VIII). Penhow was small, but furnished and with demonstrations in the kitchen and yard.

Also is St. Fagan's with small palace and lovely gardens. Right next to the palace is the Welsh folk village, a collection of houses from verious eras. Craft demos and farm animals to see.

However, near Conway, besides those mentioned are other wonderful sights. Bodnant Gardens are fabulous. In Llandudno there is a small doll museum which I enjoyed. It was also fun to walk along the seafront there. Penrhyn Castle (between conway and Bangor) is a nineteenth century recreation built by a wealthy industrialist. Interesting furnishings and displays. Bangor has an interesting cathedral with a biblical garden (Plants mentioned in the Bible). Benau Festiniog has an old steam railroad which goes down to the coast. Also there is a chance to visit a slate mine there.

And best of all, the scenery is stunning!

I forgot to mention that near Caernarvon there is a museum of Roman occupation and some Roman ruins which are interesting.

On another post someone mentioned the Castle Bank Hotel in Conway. I was glad to see that it was still a going operation. I stayed here for a week twenty years ago and used it for a base to explore all of the above mentioned places. As you have only 3 days, you will need to pick and choose, but I am sure you will love it there!

I forgot to say that I did these by bus, train and bike. However, I realize that I could have combined more things in a day, had I rented a car. suggest taking the train to Conwy and renting a car there.

do you have to get back to London to fly home? If so, consider taking the ferry from Ireland to Wales, and then after a few days take the train down to London. It would save you and extra travel day.

And if you have to get back to Ireland to fly out, I'd still consider Ireland > Wales > London, and then fly from London to Dublin or Shannon. There are very inexpensive fares on those routes.

Just other options . . . . .