Are there any strict spelling rules that determine the spelling of "ceathair"? Having forgotten the spelling, and only hearing the spoken sentence, I made a guess with "caithear". In retrospect, I assume there would be some difference in pronunciation if *caithear were a real word?
Part of my question is also the nagging sense that there is a system in place determining that you couldn't swap cea- for cia-, nor -air for -aer... But I haven't gotten the hang of the conventions and their effect on pronunciation.
‘ea’ in stressed syllables is pronounced /a/ (/e/ in some parts of Ulster) after a slender consonant and before a broad one. When the following consonant is slenderised it becomes ‘ei’ (pronounced /e/, or /i/ before ‘n’ and ‘m’), except in the word ‘fear’ which has the semi-irregular plural ‘fir’. Historically both ‘ea’ (written ‘e’ in Old Irish) and ‘ei’ would’ve been pronounced /e/ in all words where they appeared, but this changed as the language evolved. ‘ea’ is also the most common way to write the unstressed vowel /ə/ after a slender consonant and before a broad one. The same vowel /ə/ is alternatively written ‘a’ (broad—broad), ‘ai’ (broad—slender) or ‘i’ (slender—slender). Other spelling occur as well (in fact any unstressed short vowel is pronounced /ə/), but these are the most common. ‘ceathair’ is spelt the way it is because it has a slender ‘c’ and ‘r‘, but a broad ‘th’: /ˈk′ahər′/. Your proposal would have to be “ceithear” in order to comply with Irish spelling rules, but then you’d have a slender ‘th’ and a broad ‘r’ (/ˈk′ah′ər/), which would make it a totally different word (although as it happens “ceithear” isn’t actually an existing word).
‘ia’ (written ‘iai’ before slender consonants) on the other hand is a different sound entirely: a diphthong rather that a short vowel, transcribed as /ia/ (or in some guides /iə/). It’s sound is like ‘i’ in ‘hit’ followed by ‘a’ in ‘about’. It is quite similar to the vowel in words like ‘seer’, or perhaps ‘here’ or ‘fear’, when pronounced with a standard English accent (not with an Irish accent, mind you).