It's Irish, brewed at St James' Gate Brewery in Dublin. However, unless you're actually in Ireland, the Guinness you're drinking is likely to have been "brewed under license" by a local brewery. This happens quite a lot with widespread brands of beer.

Guinness is an Irish brewery which makes dark beer - 'stout'.

My own answer on Dec 19, 2014: I finally visited the Guinness beer museum in ireland and confirmed it is Irish with long history. I also checked the labels of Guinness beer in China, they are all said to be imported from ireland. Cheers.

Aussies are fond of it too:D

@James, thank you!