It occurred to me recently that the adjectival suffix -(a)idhe is probably just a form of the suffix -dha (/-da/-tha/-ta) which developed after unstressed vowels*: e.g. eagna + -dha = eagnadha > eagnaidhe (just as the plural suffix -adha became /i:/), and after consonant clusters** with an epenthetic vowel: e.g. Críost (+ a) + -dha = Críostadha > Críostaidhe.
(*With nouns which have/had cases ending in -dh the form used is -ta, e.g. file (fileadh, filidh) – fileata. **The consonant clusters are sometimes formed through syncope, e.g. talamh – talmhaidhe.
I’d imagine the development of simplidhe (etc.?) was influenced by adjectives of this type.)
Yes, broad -(e)adha would generally become /u:/ but frequently didn’t, the best example of which is the old plural suffix -(e)adha > -(a)idhe > -(a)í. Interestingly, this developed as /u:/ after certain words in Mayo (“The Irish of Erris, Co. Mayo” pg. 191).
I mentioned simplidhe as I’m unaware of any earlier form like “simpil” or “simple” to which the suffix could have been added. It’s possible the existence of adjectives ending in /i:/ influenced the form the loan word took in Irish, or that the suffix was added as it was adopted.
It could have influenced it at least, I suppose, but I can’t think of any case of an English adverb becoming an adjective in Irish. Knowing when the word entered Irish would be useful. McBain (under “silpidh“) and the DIL aren’t much help.
According to “The Irish of Erris” it’s a small group of nouns ending in /-É™/ that form the plural with /-u:/. The examples given are: cleite, faithne, gearr-chaile, gloine, leithe, reithe, seithe & sine. It does seem to be a Connacht feature, I found no mention of it in “Gaeilge Theilinn”