Even if it's a specific lunch, I would still just say "lunch". Can someone who speaks Irish well tell us: Is it more common in Irish to say "an lón" than it is to say "the lunch" in English? If it is, "lunch" should be an accepted translation in English. Translation is not about one-to-one correspondences but expressing the same meaning in whatever is the appropriate way to do so in the target language.
It would be more usual in English to say, "Lunch is over" rather than "The lunch is over". I suppose if you were talking about a special lunch (an awards lunch, for example), then you would say, "The lunch is over. And you missed it!"
In Irish, is it possible to say, "Tá lón thart" to mean "Lunch is over." If so, then I'll accept that "Lunch is over (or finished) " doesn't work here. In the meantime, reported that "Lunch is over" should be accepted (March 30, 2016.)
Technical answer: there is no "the" to correspond to "an lón"
Language interpretation: The sentence "The lunch is over" is related to your eating or not eating (either you have had your fill, or you will not be served and remain without food). The sentence "it is past lunch" would rather be a statement of time, sort of saying "It is half one" = 1:30 pm or later.
It seems we are not going to get a response to whether the definite article may be omitted when translating to English, but I think we can take it that we can do so. I recently came across the sentence "Chaith mé an lón" translated as "I ate lunch". On Memrise, I found: "Tagaim ar scoil ar a naoi a chlog, agus ithim an lón ar a haon". No special or specific lunch. Just "lunch". To be fair, I found other examples where the article was omitted in the Irish "Ithimid lón ag a haon a chlog", so my guess is that this generic use of "an" is in transition in Irish, or has always been optional. Which gives us a problem when translating to English. Without context, do we presume the general or the specific? My taste would be to go general, and that means I would render our sentence "Tá an lón thart" as "Lunch is over", though I don't suppose Duo will go with that any time soon..