I have pages on my site that are essentially descriptions of similar/identical products on other sites (not scraped content). These are on separate pages than the actual product pages (but do link from the product pages, sort of like a 'more info' type page), and therefore, are irrelevant to me as far as search rankings go. The product pages themselves rank just fine, as does the site itself.
These pages are hidden in the sitemap, but there is consistent linking throughout the site where I'm unsure if we would be able to place a nofollow on the links, along with a few obscure places where other pages link to them.
If Google does find these pages, I wouldn't care if they slapped a duplicate content penalty on them since I don't need them to rank anyways. But, on the other hand, I don't want the fact that these pages have been penalized to have an adverse effect on the rest of the site. Sort of a guilty by association sort of thing; I know how Google doesn't like you to be anywhere in the neighborhood of spam.
Not really - a true duplicate content "penalty" is a very rare thing, handed out to serious spammers. Instead what happens is that some urls get filtered out on the appropriate searches and only one url is shown. The only problem would be if one of these irrelevant started to rank and your strong page got filtered out. Sounds like that's not happening.
If you want to safeguard, you could use a meta robots noindex in the head of the irrelevant pages.