I am spending 5 days in Dublin for Tradfest in late January. I'd like to take an excursion for one day, perhaps by train and arriving back in the city for an 20:30 concert. Ideas would be welcome.
Are you familiar with, and interested in, Newgrange? The Mary Gibbons tour (a comfortable day; you'd easily make the concert) gets excellent comments here on the Forum. We greatly enjoyed it in May 2018. It includes a stop at Hill of Tara, and a drive through the scenic Boyne Valley. The topnotch commentary is fascinating. (Our guide was the highly regarded Phelan.) Weather in January might be a factor as the most interesting sites are off the comfortable bus. https://www.newgrangetours.com/ (Reserve if interested.)
We didn't get to Belfast, but I suspect some of the Forum regulars will be suggesting that as well. That might make more sense in January.
You can take the DART, the commuter train,out to Howth.