And moreover, there are some really good linguistic reasons why the Irish monks picked these particular letter combinations to stand for these particular sounds (note that this is based on a Scottish Gaelic course I took many years ago so bear with me if I get a few details wrong).
Let’s start with <bh>. Now, the Latin alphabet at the time didn’t have a letter for the /v/ sound, but it did have an alternative way of writing the /f/ sound, which was spelled <ph> when it was borrowed from Greek (for other historical reasons). Well, /p/ is a sound that’s produced by letting a burst of air out from behind your lips while your vocal cords aren’t vibrating (it’s a voiceless bilabial stop), and /f/ is a sound that’s produced by letting a small amount of air out from behind your teeth on your lips while your vocal cords aren’t vibrating (it’s a voiceless labiodental fricative). So <ph> is kind of like a more breathy <p> (/h/ is a fricative like /f/). And /b/ is the same as /p/ except your vocal cords ARE vibrating, the exact same way that /v/ is like /f/.
So <p> is to <ph> as <b> is to <bh>.
Adding <h> to a consonant to indicate a sound somewhat similar to the base letter was very common in post-Latin Europe: English, Irish, French, German, and many other European languages ended up with <ch>, <sh>, <th>, <gh>, <wh>, and so on. It just happens that some h versions are found in some languages and not others, and pretty much every language uses the h variations to stand for different sounds. (Especially “ch”).
Now let’s get to vowels. There are two groups of them: /i/ and /e/ are one group, while /u/, /o/ and /a/ are another. The traditional Gaelic (Scottish and Irish) terms for these groups are that /i, e/ are slender and /u, o, a/ are broad, but linguists also split them up, as front and back vowels.
Front vowels /i/ and /e/ tend to pull consonants along with them, in very many languages, especially /t/, /d/, /k/ and /g/. It’s a process called palatalization and there’s a whole Wikipedia article about it. So the <si> in words like “Sinead” is palatalized just like the <si> in Latin-derived words like “precision” (not to mention all the words in “-tion” and rapid speech pronunciations like “didja” and “gotcha”). Palatalization also explains why English has “hard” (=broad=non-palatalized) and “soft” (=slender=palatalized) pronunciations of <c> and <g>, which are split by the same set of vowels – compare “cat” “cot” “cut” with “ceiling” or “cite”. (The pronunciation of <g> is more complicated which is why no one can agree about “gif”.)
And English spelling also retains or adds a silent letter where it would cause palatalization confusion. Think about words like “peaceable”, “placeable”, “changeable”, “salvageable” – normally a silent “e” is dropped before -able (bribable, adorable), but it’s kept here. Or the “k” added in “mimicking”, “frolicking”, “picnicking” despite “mimic, frolic, picnic”.
Mutation (changing the initial sound of a word for grammatical effect) does seem to be particular to the Celtic branch of the Indo-European family tree, although various kinds of mutations are found in other languages.
Irish spelling looks weird if you take English as a starting point, but if you take Latin as a starting point (which it was), both Irish and English do different (but sometimes related) weird things.
