Having lived in Cardiff for a bit I'd like to think I have some ideas on where to watch it, however, a place which has opened relatively recently is http://shootersportsbar.co.uk/cardiff" rel="nofollow - 'Shooters' sports bar beside the Millenium Stadium (Across the road from Cardiff Central, 2 minute walk to Prince of Wales and Mary Street etc.). 

Obviously there's the big screen etc. but this place is walking distance to the City Stadium and can imagine that if you're going to find a ticket on the day, it will be around there. 

Let me know your thoughts...