In the centre of town there's the Lion Bar - can't miss it, has large banners outside. When you get inside it's half the size you'd expect and although a 'sports' bar only two screens on same side so go early to get a good spot. Alternatively, if you're Welsh maybe just stay in your hotel room ;-)
Some of the hotels show sports as well, not sure if they'd show Rugby over any footy that may be on but can always ask.