Years ago I worked in the Research Unit at Guinness, Park Royal, NW10.
There was no difference between the two breweries brewing methods, except that obviously there were differences in resourcing of the raw materials. he intention was always to produce a consistent product, wherever it was brewed.
It is a natural product, so there will be some slight variations, but generally, I think most people would be hard put to tell the difference in a taste test. It was often said that the turnover was higher in Ireland than the UK so maybe there is something about how long the keg has been open....
There is always the environment effect as well - There was another thread which talked about how the .local drink tastes terrific when you are on holiday, but the same drink tastes rubbish when you get home, so maybe that has something to do with it!
So just enjoy and don't worry about it!