I will ask our Devs if they can add that, but it may not be feasible, nor would there be any way to enforce that -- i.e., a Northern Irish buyer could still just select Great Britain and input their NI shipping address details anyway, so you'd still need to watch for any such orders and refund them.
Our list of countries for shipping (and tax) calculation just replicates what the US Postal Service supports for live-rate lookups, so most likely any "Great Britain (excl. Northern Ireland)" we could add would have to be just a differently-labeled but functional-duplicate of the existing "United Kingdom (Great Britain and Northern Ireland)" entry.