your best bet is probably to find the details for the documentary in question and get in touch with the researcher who worked on it through TG4.
Just checked the reference in the UCD catalogue here - http://www.ucd.ie/library/special_collections/archives/modern_lit/oriordain.html which you've probably seen.
Sometimes with archival material, the library will send digital photos or photocopies to overseas researchers for a fee, or the manuscript is available on microfiche, but that doesn't seem to be on offer for this collection. In any case, you'd need to know where in the 37 years of diaries the quote comes from, so asking the researcher who worked on the documentary would still be the simplest way in.
Not much of the diaries seem to be published though there are sections on this Gael linn recording - http://music.napster.com/se%C3%A1n-%C3%B3-r%C3%ADord%C3%A1in-music/album/fil%C3%ADocht-agus-pr%C3%B3s/13548804. Still, you could check whatever the standard biography of him is, just in case the documentary researcher got the quotation from a published source.