Lots of people already have the idea. That and the city is over crowed, making Busking in Galway a tough way to make your money. I suggest you find a galway girl who can support you and who has an apartment and who can support you if your busking doesn't pan out.
However, the city is unique in that it allows, and can support, not just buskers but proper street performers, guys juggling chain saws and doing stunts on unicycles, and the like.
If you're a guy with an acoustic guitar looking to make money, play on shop street either opposite Knock-tins (this is a phonetic spelling) or just up the street from the chip shop near the big book store.
Play during early evening or at night and play popular songs. The more popular the better, and if you're a terrible singer and you play popular songs, that's ideal. Galway locals love to see a guy making an ass of himself as long as they know the tunes, and they're happy to throw you a Euro, 2 or more if they're drunk.
How late can you play? Usually the cops will shut you down around midnight unless they enjoy what ever you're doing or if they know you.
The other option in Ireland, in Galway at least, is to busk during the day. It's tough money. I usually make about 8 or 10 euro an hour playing during the day but that's in Buttermilk Lane and I'm not playing popular tunes. If you're going to play on shop street during the day you probably should think about using either an amp or a woman to sing and dance for you. I don't make much money during the day but if you're a novelty act, like Rachel, the Canadian woman who sang with her cello, or, whatever, if you're novel you can go busking during the day and have success. Good Luck - jesse