Well, I was listening to some Gaelic music on youtube just now, and thinking about the language's situation (extinct as a native language in the bulk of Ireland, with native speakers a few percentage points of the population and falling), so the thought occured to me-what POD would be necessary to get Irish Gaelic in the same situation as Welsh? That is, spoken natively by around 25% of the country (with that number remaining fairly stable), having a widespread prescence in all parts of Ireland, and generally not in immediate danger of dying off within the next 50 years or so.
(Irish Gaelic as a majority language would be interesting, but I'm assuming a POD after 1800 or so, so I think that would be very hard to have work realistically).