I just read the book. Basic biology is somewhat lacking throughout. But potatoes are one of the few foods you can survive on indefinitely. They contain quite sufficient vitamin C to prevent scurvy.
Sufficient light to grow 'em would have been a problem, but if they can get by well enough to feed a nation even with Ireland's average cloud cover, perhaps a better choice than most crops. Might get one somewhat scanty crop, anyway. (I've seen 'em produce even when all the light they got was what leaked through broken boards into a closed shed.)
The bacteria issue was overblown; Watney could repopulate the whole place from his own colon, even if a large proportion didn't encapsulate as many bacteria do when stressed. And potatoes themselves are hardly sterile.
I did gather the author has never used freeze-dried food, including instant mashed potatoes.