If you are not sure where to start then head downtown to Wind Street which has loads of great pubs that welcome Stags having a night on the town. It is a great street to have a pub crawl as there are at least 7 pubs within 200 yards including the La Prensa. Other starting point is the High Street with Chanpers, The Kings, and La Viva.
For Nightclubs head out to The Kinqsway where Flares and Oceana are probably your best bet. At the weekends both are open to after 3am but check to confirm. Every Stag Night out has to have at least one trip to some form of Lap dancing club. Try Studio 95 on Mansel Street that receives 5 star reviews time and again.
Have a great night out and try to remember your Hotel name- it makes getting a taxi back there so much easier