Let me offer one bit of information that is relevant to this project.
Last month I was browsing TG4’s site and came across a statement (which I can’t relocate) that they were actively working on an overhaul of programme subtitling. There wasn’t a lot of detail but the piece clearly suggested the big change they would be to offer different subtitling options as in:
a) without English subtitles
b) with English subtitles
c) with Irish subtitles
If these subtitles are offered as different elements within the broadcast stream then theoretically it would be possible to extract them and convert to the various subtitle formats (srt. sub. etc). Then it would be possible to watch TG4 programmes and switch ‘on the fly’ from none, to English or to Irish subs.
One could also choose (as I sometimes do with difficult German films) and display both (Irish frame bottom, English upper frame).
One warning is due though though, when I read the information on their site, I noted the page itself had no date of issue nor any start date for the feature. So further clarification is needed , maybe from TG4 itself? Sorry I can’t point to the page on the TG4.