Some interesting myths about Guinness that need to be dispelled.

"Some time ago, at the main Guinness brewing plant in Ireland, the vats were opened and drained to clean them out. Once empty, the bottom of the vats were found to be full of dead rats. Of course, as soon as this was discovered the vats were cleaned and sterilised. However people started complaining that the Guiness didn't taste right, and so now Guinness is filtered through lamb's blood, which goes some way to explaining the unique taste. (I was told this, and it may not be true."
- This is not a true story. Guinness is most certainly not filtered through lamb's blood, neither were dead rats ever found at the bottom of vats as St. James' Gate.

""Dublin makes five or six principal versions of Guinness, in a total of 19 variations, and exports around 40 percent of its output."

- Dublin makes two versions of it's "Guinness" product, in a total of about three variations, packaged and distributed in a variety of different forms. The Guinness brewery itself brews many other products that aren't Guinness.

"The Guinness you buy in bottles is different to the Guinness in cans and on tap. The Guinness in the bottle has live yeast in it, and in America is called 'Guinness Extra Stout'. The cans and draught are simply 'Guinness'."

- Guinness is available in bottles in both forms: Draught Guinness in a bottle and Guinness Extra stout. They are two different products. Both products are available in cans and bottles. Guinness Extra stout is not available on draught.

That takes care of the blatant untruths.
Now for the misunderstandings.

The Guinness brewed in Dublin is not "better" than Guinness brewed in London.

Despite what your pub genius buddy may have told you, and taking into consideration his in-depth knowledge of quality control within the brewing industry, the product brewed in London is identical to the product brewed in Dublin.

But everybody knows that the Guinness in Ireland is better than anywhere else!

This is bullshit, an urban legend almost. When Guinness is brewed and ready for distribution, even the most acclaimed brewmasters cannot tell the difference between the "geography" of the product. Bear in mind that these guys can identify minute traces of chemicals in beverages and estimate their quantity... by taste alone.

The reason Guinness tastes better in Ireland, correctly pointed out by ryano, is the way the product is managed when it leaves the brewery or distributor. In Ireland, thousands of pints of Guinness are consumed every day, therefore the volume of Guinness coming though an Irish pub is much greater than an equivalent English pub. This means the kegs are changed more often and the lines are cleaned more often.

This equals fresher and better tasting Guinness.

Because less is sold in the equivalent English pub, the stout is lingering in the keg longer, becoming less fresh, giving it that distinctive bitter taste. The Guinness lines in the equivalent English pub would not be maintained as well as the Irish pub either. These factors, plus illiterate bar staff who don't know what they're doing, all contribute to the Guinness tasting bad in England sometimes.

Don't get me wrong, I've had some lovely pints of Guinness in London, I've also had some really, really shitty pints in Dublin, including a pub next door to the brewery itself.

It has nothing to do with the Thames, the Liffey, rats or lamb's blood.

It's about volume and attention to detail.

Sources: My father worked for Guinness for 32 years, 20 years R&D and quality control, 12 as a director. He'll give you a clip if you don't watch your manners.