Upvote:1
From having been in Dublin a few times, I agree with the other answer that wandering round the popular area will give you the best 'packed to the gills' pubs.
To find which areas are the most popular at this time I would look up the online events, if there are three places advertising in the same street, it is a good bet there is more going on.
I also used printed guides (if was a few years ago) and the Lonely Planet information was (then) spot on. These days I would go to their site, and maybe forum, to see what they have on Dublin.
And ask around. Hotel and hostel staff are mostly aware where the good places are. Tourist information offices have the information and random student aged youngsters on the street (look for clothing styles you would expect in your favorite clubs) can be good sources of information as well.
Upvote:2
If pubs have an "event" such as live music or a quiz for example, it is often mentioned online, on flyers/posters in that pub, and maybe in other places too. Just keep your eyes (and ears) open while wandering around the city.
In my experience, live music in pubs is not hard to find in Dublin, or many other places in Ireland.