The Great Hall The Great Hall is Cardiff Union's largest venue with a capacity of 1500 and can be accessed from the entrance on Senghennydd Road. One of the oldest Live Music venues in Cardiff, The Great Hall has played host to some of the biggest acts in the world. Y Plas As a live music venue Y Plas has a capacity of 1000 or 1200. It can be accessed via the steps at the front of the Union on Park Place. It is also where our clubnights take place with the addition of two other rooms. Y Plas has one large bar in the main room, a smaller bar on the mezzanine balcony and two bars in the other adjoining rooms. There is also a cloakroom in the back room.