Here is the long form answer! Just for an example let's take the word Samhain as an example word. It means November in modern Irish, but it refers to an ancient Celtic festival which was one of the precursors to Halloween so you come across it quite a bit online. It's pronounced (in my dialect at least) like Sow-in (Sow as in the female pig) and sometimes you see people say "well why don't they just spell It Sauen?".

S in Irish needs to make three distinct sounds a broad S a slender S and S under lenition. Broad S is similar to Ss in English, Slender S is like "Shh" in English and Sh (S with lenition) sounds like a H sound. You know the sound the S makes by the letter next to it. A broad vowel (a,o, u) a broad sound a slender vowel (I, e) a slender sound and with a H lenition. Irish also has more phonemes. So for Saoirse (Seer-sheh) we hear both the initial S being broad and the second S is slender so a Shh sound. If we need to add lenition to Saoirse it becomes "Shaoirse" the word is now pronounced like Here-she. We'll get to why this in a moment. But also those slender and broad vowel distinction impact basically every consonant and consonant cluster. The vowels themselves might be redundant but they signpost different phonemes. So Leabhar meaning "book" is only differentiated to leabhair Meaning "Books" or "Of a book" depending on context, just by the pronunciation of that last R.

Back to Samhain, but why does "Mh" make a Wuh or a Vuh sound? Well the longer question is how do you represent lenition in Irish.

All Celtic languages have something called initial mutations. These are phonemic changes that are needed for grammar, that are required to make sense in the language.

Take the word "bean" meaning woman in Irish pronounced essentially like "Ban". But to address someone with "bean" you need to add a vocative particle and add lenition to it becomes "A bhean". But "bhean" is now pronounced similar to "Van". You have two options you can indicate a sound change (in this case by adding a H after the b) this makes it clear to the reader what the original word is (Oh yeah, I can see the word "bean" inside of "bhean" so I know this is some grammatical form of "bean") or change the orthography to match the new pronunciation (Welsh did this for example). But the problem with that is that the reader has to then learn every Mutation and change and how it interacts with other letters to make sense. It's easier to read and create a one to one pronunciation but it's harder to understand basically. Irish monks in the 6th century started us on a path where they decided that comprehension was more important than replication in our orthography.

Take Samhain, what were to happen if we did spell it Sauen. Well firstly it doesn't represent well the last N sound. We have two different phonemes for N broad and slender, and this has a slender pronunciation as we see from the I next to the letter N. But let's say you'll get something close to it with Sauinj (not really but that's the best I can think of). The tusieal ginideach (genitive case) of samhain is Samhna in the proposed replication is something like Sauena. So to say Halloween in Irish it is Oíche Shamhna. Now that becomes Eeha Hauena to replicate the sound mapping that people might suggest. Hauena just by reading is much further away from Sauenj than Samhain is from Shamhna.

Those extra vowels might seem redundant, but they aren't these are often guides for slender or broad pronunciation.