There is a park in Dublin called St Stephen's Green. Which prepositions of place would you use (and what context would you use them in) in the sentence "It's raining ... Stephen's Green"?
The position in Britain would be as follows:
If St Stephen's Green was, let's say, a village on the Yorkshire Dales (or village, town or city anywhere) it would either be in or at. The larger the place the more likely it is to be in I believe. One would be unlikely to hear Is it raining at London/Paris/New York ? etc.
If St. Stephen's Green was a street, or a square in London, let's say, it would almost always be in (though it could be at). However if it was a very large and important street, one might use on. Is it raining on Knightsbridge/Piccadilly/Bayswater Road? would be perfectly usual. One might also ask of someone on a journey Is it raining on the M4 motorway?
If St. Stephen's Green was a large city park - it would be in. Is it raining in Hyde Park?. However if it was a very large expanse of open country one could use on. Is it raining on Hampstead Heath? for example. Though if it's a forest it would be in. It is raining in Epping Forest. But open country would always be on. It is raining on Dartmoor/the Salisbury Plain etc. In counties or regions it is always in; raining in the Peak District/the West Country/Somerset etc. Although it does rain on the Norfolk Broads.
If St Stephen's Green was a railway or London Tube station - I think it would almost always be at.
To answer the very specific question about St Stephen's Green in Dublin - if it is what I suspect, a city park - it would almost certainly be in. However if it is a very large expanse of country it would most likely be on. It would not be at.
However this question may only be answerable with reference to local knowledge. If, as has been mentioned by one commenter, that they hold Concerts on the Green (at St Stephens Green), then I don't see why it could not rain on St Stephen's Green. It certainly rains on Clapham Common
I hadn't realised, until I began to think about this, how nuanced it all is. But it is the sort of thing that picks out native speakers from otherwise fluent speakers. Choice of prepositions also picks out Americans from British, in a very big way.
In my opinion, all three can be correct - it simply depends on what you're attempting to say, and how you're viewing "St Stephen's Green". (disclaimer - I'm an American.)
"raining on St Stephen's Green" - This is expressing the concept that raindrops are hitting the physical substance of St Stephen's Green. That is, "St Stephen's Green" is the actual grass/earth/pavement, and rain is falling on that physical thing.
"raining in St Stephen's Green" - This is expressing the concept that "St Stephen's Green" is a geographic area with extent - more specifically that there is a three dimensional space which can be labeled "St Stephen's Green", and that rain is occurring within that volume. The volumetric aspect of the green can be a little fuzzy, given that there isn't necessarily a defined "top" to it, but at some point you go from "birds were flying in St Stephen's Green" to "planes were flying over St Stephen's Green".
Even if you're thinking about "St Stephen's Green" as a two dimensional extent - for example, a region on a map - "in" may be appropriate if you're also thinking about the rain occurring in that same two dimensional context. For example, a TV weather forecaster with very detailed forecasts might point to the weather map and the storm clouds icons within the boundaries and say "today it will rain in St Stephen's Green".
"raining at St Stephen's Green" - This is expressing the concept that "St Stephen's Green" is a location without geographical extent. That is, "St Stephen's Green" is a point location and rain is occurring in a region nearby.
Note that the choice of preposition doesn't necessarily have to do with the intrinsic nature of the location, but more with how you're conceptualizing the space. "The naughty child was playing in Piccadilly Circus." but "I'll meet you at Piccadilly Circus." and "Protesters drew graffiti on Piccadilly Circus.": you go from Piccadilly Circus being a space within which people can exist, to a single location where two people can meet, to physical surface which can be painted.
Certainly the type of location would influence the choice - larger areas would more likely be an "in" than an "at", for example, being harder to conceptually reduce to a single point. Frequency of usage also tends to make certain pairs sound better than others. But intrinsically the choice of preposition is contingent on how that space is conceptualized, rather than any innate nature of the space.