All British Guinness is the same as it all comes from the same brewery.
The perceived taste difference is mostly psychological, but is in some part related to the way it is served, and the actual amount a particular premises sells, Guinness has a specific way that it should be stored and served, not all low volume sellers keep up to standard.
Until recently, (2005), Guinness was brewed in Dublin and the original brewery
at Park Royal in London (this is the original location and not Dublin, Arthur Guinness started brewing with a grant from the bishop of london).
The colour and flavour is accidental and derived from an incident where the grain was burnt and as there were limited funds, they were used anyway.
Guinness is not Black, by the way, but Ruby Red, (hold a glass up to a bright light, before you drink it, and prove it for yourself)
At 185 Calories per pint guinness is less fattening than many Light Lager beers
I'm actually a fully qualified Guinness Trainer - so the above information is direct from those in the know and not Wikipedia which is a little confused.