I dunno if this will help (it helped me), in Hib Eng;
'(it) is a teacher he (is)' - 'Is múinteoir é'
a teacher is he - Múinteoir is ea é.
Both are 'he is a teacher' in standard English but the second is slightly more empathetic.
I'm guessing 'HE is a teacher' wud be something like; 'Sése is ea múinteoir é.'? ( Is sése féin múinteoir atá ann?)
It's the same word, so by default pronounced 'iss'. But before é, ea, í, iad, it's 'ish'. https://en.wiktionary.org/wiki/is#Irish
As an add on: especially in rapid speech, the DeNTaLS DoTS consonants tend to change their broad/slender pronunciation to match the first letter of the next word. So is mise tends to be 'shmisha', while is túsa is 'stoosa' (the 'i' is often dropped, as acknowledged in writing for sea). Same for other words ending in a DeNTaLS DoTS. https://en.wikipedia.org/wiki/Irish_phonology#Sandhi