I agree that 'irrelevant, academic' is now clearly the basic, primary meaning, that it's not at all wrong, and that it's by no means restricted to US legal jargon. Where the Concise OED got that idea I can't imagine; other dictionaries all correctly show it as a perfectly normal sense.
Nick is (apparently) a voice for AusE, and several of us have concurred on AE. It might help to get a little more feedback from BE speakers on how many of them still use the older meaning at all, and if so, how often. So far Anne is the only vote for it, and IIRC hers is a rather conservative legal milieu. I found it interesting that in the worldwidewords.org link Mattes cited, Michael Quinion, a BE speaker, does not in fact say that the historical sense persists, but simply that the meaning has now changed: '...we’ve seen a curious shift in which the sense of “open to debate” has become “not worth debating”.'
In fact, it can be surprisingly hard to find umambiguous modern attestation for the historical 'debatable' sense. For instance, the NOAD undercuts its case by citing as an example a sentence about the greenhouse effect that can easily be read instead with the more modern 'irrelevant' sense. In fact, that's indeed how I believe most speakers would interpret it, i.e., expecting the next sentence to be something like 'What mattered now was finding a way to protect low-lying areas from rising sea levels caused by the melting of polar ice caps.'