There are a few different beers that use the name Bass. The kegged and canned one you get here is essentially an Irish red, now brewed by C&C in Glasgow having previously been brewed by Interbrew in Belfast before that closed.
The cask Bass that the gammon get misty-eyed over is a bitter brewed by Marstons for AB InBev who own the brand. ABI also make a different bottled Bass at their own brewery in Salmesbury, mainly for export to the US but I think it's sold in GB too.
And then there's a Belgian-recipe Bass, which I think comes from Salmesbury too these days. There may be more.
Last time I saw it in a bar was about a month ago and that was in a hotel in Cookstown. Some of the multiples in the North carry it in bottles and it's cheap enough; it's ages since I chanced it though.