Hi. I would love if anybody could answer these questions. According to omniglot the diphtongs ua and ia are pronounced with a shwa vovel instead of a. I presume they previously were pronounced with an a. Are there any dialects left where they are still pronounced with an a?

About the leninitions bh ch dh fh gh mh ph sh and th. I can see what they are pronounced like today but what was their original pronounciations?

Still heard? Yes — Ulster Irish (Donegal) often keeps a more open Drift Hunters vowel, closer to the original.

bh, ch, fh, gh, mh, ph, sh were pronounced as today.
bh, ch, gh, mh, ph were pronounced like b, c, g, m, p but without an oral stop of air flow, i.e. as corresponding fricatives.
sh as /h/, fh silent because s and f are already fricatives. They lost frication.
mh was a nasal /ṽ/, bh a simple /v/. (broad /w/ is probably a later innovation)

dh, th were pronounced as d, t without oral stop of air flow, that means as dental fricatives, like English th (dh voiced /ð/, th voiceless /​θ/). There were probably different broad and slender versions, too.

bh and mh and ph (and also f) were pronounced bilabially, meaning they were pronounced by making contact between both lips as opposed to the English sounds, where the top row of teeth makes contact with the bottom lip. The whole idea of lenition is that the place of articulation remains the same (except for f --> fh and s --> sh), but the manner of articulation is 'softened', meaning that the amount/force of contact produced is reduced. While this 'rule' has become less regular in modern Irish (see the pronunciation of th and dh), in the older language it was consistent (again, with the exception of fh and sh, which Labhrás has explained above). Hence, bh, mh, and ph were pronounced by making contact between both lips (so, in narrow transcription the symbols /β/ and /ɸ/ would be better at describing these phonetically than /v/ and /f/, but you don't really need to worry about this), as the consonants b and p are pronounced making contact between both lips (in both Irish and English) as opposed to between the upper teeth and the bottom lip. This is also the reason that dh and th used to be pronounced as /ð/ and /θ/ respectively (like in English 'this' and 'think'), because the consonants t and d were pronounced by pressing the tongue against the back of the teeth.

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I'm an intermediate speaker of the Corca Dhuibhne dialect of Irish and also have knowledge on the old spelling
Soir gaċ síar, fé ḋeireaḋ thíar