I had a lucky escape on the Ramblas a couple of years ago, was using an ATM when a smooth operator appeared from nowhere, pretended to be helping me read the Spanish instructions on the screen (when they were in English!), and before I knew it he had my card half hidden in his hand. I grabbed it from him and he calmly walked away.
That's as likely to happen me at any ATM in Dublin, but I'd be wary enough, I've heard enough tales of woe from people who've visited Barcelona to realise this carry-on is common enough.
A tip: I'd avoid the Ramblas, horrible dive of a street, full of tacky shops and crappy stalls and usually heaving with tourists. Down by the harbour area is beautiful, great restaurants, cafes, etc.
Have a great time, it's a wonderful city. (Make sure you take a tour of the Nou Camp!).