One thing which ligeadóirí agus casadóirí an Chaighdeáin – the makers of standard Irish – got wrong is the treatment of the preposition i. Many learners are confused about what the exact relationship between i and sa is, because sa does not look like i at all. If the caighdeán had kept ins an, ins na instead of introducing the colloquial form sa, sna, I think learners wouldn’t be even half as confused.
So, just as an introduction:
When we take the preposition i and add the definite article an or in plural na, what we get is ins an, ins na. Thus:
i gcathair ‘in a city’ but ins an chathair ‘in the city’
i dteach ‘in a house’ but ins an teach ‘in the house’
in eitleán ‘in an aeroplane’ but ins an eitleán ‘in the aeroplane’
in áit ‘in a place’ but ins an áit ‘in the place’
i gcathracha ‘in cities’ but ins na cathracha ‘in the cities’
i dtithe ‘in houses’ but ins na tithe ‘in the houses’
in eitleáin ‘in aeroplanes’ but ins na heitleáin ‘in the aeroplanes’
In the caighdeán we are supposed to use sa, san, sna instead of ins an, ins na:
sa chathair, sa teach, san eitleán, san áit, sna cathracha, sna tithe, sna heitleáin.
This is a common reduction of ins an, ins na in the spoken language, but for a learner, it is not easy to understand why such very different words as i and sa could be related. If you think of every single sa, san, sna as ins an, ins an, ins na, this should help you.
As regards the initial mutation after i and ins an, the preposition when not followed by definite article eclipses the noun. After ins an, the noun is lenited. If it begins with s + vowel, s + l, s + n, s + r, then the standard language supposes that you prefix a t- to the s- instead, if the noun is feminine. In plural, the noun simply adds h- to an initial vowel and does not change an initial consonant at all (i.e. there is no difference between plural noun after ins na and after just na).
In dialects, the rules may be different. So:
* In Ulster, the t- prefixation to a lenitable s happens always after singular ins an. Thus, ins an tsaol, although saol is masculine.
* In Connacht, ins an eclipses rather than lenites. Thus, ins an mbaile, ins an gcathair. But it does not eclipse t- and d-, of course (neither does it lenite them, in other dialects)
* In Munster, ins an generally lenites, but there is a handful of nouns it does not lenite: ins an mbreis, ins an mbliain, ins an méid. At least in some dialects in Munster, there is an additional rule that ins an lenites other initial consonants, but eclipses f-: ins an bhFionlainn rather than ins an Fhionlainn. Many non-Gaeltacht speakers seem to have adopted this particular rule, even though they don’t generally attempt to prefer Munster words and forms.
Now, let’s add another rule. If you now want to stick to the Caighdeán, substitute sa for ins an before consonants, san for ins an before vowels and fh- (ins an fharraige > san fharraige), and sna for ins na. (Note that sa for ins na – such as sa háiteanna, sa heitleáin – is Kerry dialect, not standard language.)