Go mbeannaí Dia daoibh!

I wondering is there any pattern or rule to the preposition before certain placenames in Connacht, i.e when does one use 'ar (an)' or 'i/sa(n)'.
For example:
Ar
ar an gCeathrú Rua, ar an gClochán, ar an Tismeáin.

I
sa Spideál, i gCathair na Mart, i nGaillimh, in Indreabhán, in Inis Oírr, in Iorras.

Do you have more examples of placenames with ar before it?

Few more I've thought of while going along Cois Fharraige.
ar an => Tulach, Teach Mór, Lochán Beag, Coill Rua, Cnocán Glas

some others that take ar an before: Caorán Beag, Ard Thoir, Ard Thiar

I asked an Irish teacher from Ceathrú Rúa about this once. She told me "that's just how it is, you'll just have to remember it".

The best I can do by way of working out a pattern is to say if the name is a reference to a hill or ridge, that seems to make it a more likely candidate for use of ar.

Conversely, and perhaps counterintuitively for English speakers, where in English we typically say on in reference to islands (at least, small ones) the pattern in Irish always seems to be to use i(n) in this use case.

It seems to work like this in the three dialects:

- If the place-name starts with the sing. article ‘an’, apparently both ‘ar an’ and ‘sa(n)’ can be used, dependent on local, individual, or generational preferences, I suppose. My own impression is that ‘ar an’ is the more “native” version. Anyway, if you google around you’ll find ‘ar an Spideál/sa Spideál’, ‘ar an gCeathrú Rua”/ ‘sa gCeathrú Rua’ etc. etc. For ‘an Róimh’ and ‘an Bhruiséil’ I didn’t find any with ‘ar an’- only ‘sa Róimh/Bhruiséil/mBruiséil’.
(Only ‘sa(n)’ is used, of course, for countries/counties starting with ‘an’).

- If it starts with the pl. article ‘na’, then it’s ‘sna’. So, ‘sna Rosa’, ‘sna Doirí Beaga’, ‘sna Déise’ etc.

- For place-names which don’t start with the def. article - i.e. the vast majority - then it’s ‘i(n)’.