My wife and I talked about your trip and came up with a few things. Most of these are just "stop and go" places where you will only spend a few minutes up to maybe an hour or so at most. (other than the pub, where you need to stay for several pints)
If you're traveling up the A25 from Dublin to Belfast in the afternoon/evening hours, stop into the Thierafurth Inn and have a few pints. It'll be good craic.
Thierafurth Inn
Close by, on Moneyscalp road, is the Drumena cashel. Not a big attraction, but quite old. Built prior to 325ad, approximately.
Drumena Cashel, Down
Just a few miles past there is the town of Castlewellan. Stop in to see the 19th century castle and the forest park.
Castlewellan Castle
Castlewellan Forest Park & Peace Maze
Not far away is Dundrum Castle. 13th century. Mostly ruins now.