Someone said :
Two cans of Guinness is far too much. You'll ruin it. Don't use any at all, and use beef stock instead.
If you really want to use beer, use half a can and bring the pot to the boil for a few minutes after you've added it.
A good alternative to beef is neck of lamb. It makes its own stock while stewing. It's very cheap and can be got from a butcher. Don't put beer in it.
Yeah I've never used Guinness or any stout in mine. I do use a couple of oxo cubes. Had it in restaurant with O'Hara's and it was tasty though. Added wine once to try make a beef bourguignon.