Editing Openai/69186d20-2d14-8013-b9c6-c09000f173f6 (section)

=== Assistant: 2511132109ET ===
251113_2109ET

short answer: pretty much the same: /ˈɪʃcə/.

Reasoning with the spelling isce:
* Letters: i s c e
* First syllable must have a vowel → i is that vowel, and in a closed syllable before a cluster it’s normally /ɪ/, not /iː/. - cf. istigh /ˈɪʃtʲiː/, iarracht vs iar- etc.
* s sits between i and e, both slender vowels, so s is slender → [ʃ].
* c is followed by e, a slender vowel, so c is slender → [c] ~ [kʲ].
* Final e in an unstressed syllable → /ə/.

So an Irish reader seeing isce for the first time would almost certainly map it to:

: 

i.e., no real change from ordinary uisce in Standard/Connacht-type pronunciation.

The current u in uisce is orthographic/historical: it helps encode that old “u-ish” quality and fits older stages like [ˈuskʲe ~ ˈiskʲe], but synchronically you could absolutely imagine isce as a regularized modern spelling with the same sound.