Avoid temple bar. Take the Dart south – go to Sandycove, and go for a dip in the forty foot. The trainline is right along the cliffs, and the ride is beautiful. (You could take the train down to Wicklow/Wexford if you’ve a bit more time to kill)
If you’re after a nice pint of Guinness, Baggot st has a few nice bars – Doheny & Nesbits, the infamous Dubliners’ pub O’Donoghues, etc.
Wicklow mountain area is gorgeous. The Dublin “mountains” are even closer, and provide a great viewpoint for the city (getting to them is a bit of a pain without your own transport)
Guinness Brewery is good. Jameson brewery good too. Trinity college is popular. Phoenix park is massive. (you can rent bikes and cycle across it)
Recently there was a bit of a stunt where there were caricatures of our Prime Minister placed in two of the big galleries, but unfortunately they’ve since been removed. Most museums/galleries etc are free in Dublin if you’re into that kind of thing.