There's an Irish pub called Rathlins directly at the Schloss! Look for the crossroads at the Landgericht, the Rathlins is opposite to the Landgericht building.
I'd recommend The James, a brilliant British Pub at the Hörster Strasse. It's a five minute walk from the concert ground though. Check out http://www.thejames.de
And as I said, after the concert I recommend the Sputnikhalle with lots of 80s Indie/(Post-)Punk tunes!

