Isn't the problem more those occasional solar flares than steady-state cosmic radiation? All longterm spaceflight so far has been inside the van Allen belts which offer quite a bit of protection (for us on earth too).
The solar flares' radiation is relatively easy to shield against. You can arrange your food and water supplies to provide temporary shielding. Even a very small amount of shielding (like simply being inside a spacecraft) will stop the worst effects. Here's a graph showing some really bad solar flares and how aluminum (which is a terrible shielding material compared to food and water) reduces the dose: http://www.bioedonline.org/slides/content-slides/space-life-...
