Well, Canada's been irradiating foods for years with no apparent ill effects. Perhaps there are leads there?
As an interesting aside, my boyfriend (who grew up in Toronto) has noticed that the potatoes we get here in Chicago never, EVER sprout eyes. They shrivel up and mold, but never sprout. On the other hand, the potatoes his dad gets in Toronto (which were irradiated) will never mold, but often sprout eyes within weeks of purchase; at least if they'd been sitting in the greengrocer for a while previous. Makes me wonder what we put on our taters down here that they don't up there ...