Beer is not particularly bad for you. It's made of cereal and hops and contains plenty of essential vitamins and minerals including iron ; indeed there are some (highly optimistic) moves to classify it as a food and avoid VAT. There's also plenty of research to show that wine, especially red wine is also very healthy. And as scubadiver points out, Guinness is particularly rich in iron.
So any beer drunk in moderation is healthy - the key word is moderation. Of course, if drunk in excess, the alcohol content of beer (including Guinness) or wine will have a damaging effect on the liver.
Lager does not have the same health-giving properties - unless you're drinking German or Czech beer that conforms to the Reinheitsgebot laws. Most mass-produced lager contains a cocktail of chemicals that has little positive effect on the human system.