Already suggested on Facebook is the zoo. (it has discounts for groups, students, people over 60, people with disabilities)
Post up here!
Dublin Sketchers get together every Sunday afternoon in Dublin to sketch. We go to a different place each week. Check here before hand to see where we are off to. Come along if you fancy it! We start sketching from 2pm and meet at 4pm for a drink and chat. We are part of the global Urban Sketchers community.