The guide to ticketing on the RUI site (
http://www.railusersireland.org/pass.../ticketing.php) states that Dublin-Limerick tickets are valid on the Dublin-Cork line.
I wasn't aware of this (or had forgotten), and when I presented myself at the Cork ticket office with my Dublin-Limerick return ticket and a stated desire to leave from Cork, they told me (after a bit of head scratching) that I had to pay €11 extra (seemed fair enough to me at the time, given the fare differences between Cork and Limerick). The ticket I got in exchange had "Intercity Extension" on it.
So either the info on the website is wrong or IE just screwed me for €11 -- just thought it was of interest either way.
(PS this is not a criticism of the page on the RUI site as I think it's great and far better than the crap info given by IE)