I am not categorically against making written Irish more similar to spoken ditto, but one of the things I do dislike about the Caighdeán is how we now are supposed to use sa, san, sna instead of ins an, ins na. The fact is that learners (including yours truly, back when I still was a learner) find it exasperatingly difficult to understand the exact way how sa, san, sna are related to i, in – it feels as though there were two different words for “in”.
So, here’s what you need to know:
i + an > ins an (and in the present standard language this becomes sa before consonants, san before vowels)
i + na > ins na (and in the present standard language this becomes sna; using sa instead of sna is Munster dialect, not standard language)
When i precedes a noun beginning with a consonant, that consonant is eclipsed: i dteach, i mbaile, i mBaile Átha Cliath, i gCeatharlach and so on. When it precedes a noun beginning with a vowel, it becomes in: in Éirinn, in Amhrán na bhFiann and so on. Note though that we often see stuff like i n-Éirinn or i nAmhrán na bhFiann. These are just non-standard ways to spell it.
In the standard language, ins an (i.e. sa) lenites the following consonant: ins an bhaile (sa bhaile). But not if it is d or t: ins an doras (sa doras), ins an teach (sa teach). As regards s-, it takes the prefixed t- if we have a feminine noun there, but is not affected if it is masculine: ins an tsráid (sa tsráid) but ins an tsaol (sa tsaol). Thus in the standard language, but spoken dialects are a different story:
- in Ulster Irish, there of course always is lenition, but as regards the initial s-, you also always add the t-, masculine or feminine: ins an tsaol, ins an tsráid
- in Connacht Irish, ins an (sa) eclipses rather than lenites: ins an bhfeilm (sa bhfeilm)
- I am not sure about Munster Irish, but there is a quite common convention among non-native speakers to lenite everything else but to eclipse f-: ins an (sa) chathair but ins an (sa) bhFionlainn, and I have the impression that this comes from Munster. As regards the initial s-, my impression is that the t– is only used with certain nouns after ins an (sa), above all slí: ins an tslí, sa tslí.
Now, somebody asks here, where this leaves the book title An Chaint sa tSráidbhaile, by the celebrated Connacht author Breandán Ó hEithir, who spoke a Connacht dialect natively. Of course, it should be An Chaint ins an Sráidbhaile (i.e. An Chaint sa Sráidbhaile). I guess the use of t- here is influenced by sráid, which is feminine (thus, sa tsráid, ins an tsráid). If you ask, how the hell you are supposed to know this, the answer is: you aren’t. You are supposed to start by learning a set of rules which are acceptable by most native speakers (i.e. the Caighdeán), and then study as many original sources (i.e. books written by native speakers) as you can, until you are confident enough with the language to imitate the irregularities of native speakers.
As regards the plural – ins na (sna), it is a no-brainer: it adds h- to an initial vowel (ins na háiteanna, sna háiteanna) and nothing whatsoever to an initial consonant (ins na cathracha, sna cathracha). And as I pointed out, Munster writers might use sa in plural (sa háiteanna, or maybe sa háiteacha), but that is not standard Irish.