Couple of points:
First, Irish spelling is highly regular - much more so than English. Somebody unfamiliar with English orthography may have great difficulty with “George” or “Hugh”, for instance, and of course the British are famous for having both forenames and surnames whose pronunciation is hard to infer even if you are familiar with English orthography (Ralph, Zoe, Chloe, Isla, Phoebe, St John, Dalziel, Home, Menzies, Colquhoun, Farquhar, Ayscough, Beauchamp, Bagehot, Berkeley, Colcolough, Nighy).
Irish became a written language well before English did, so Irish spelling conventions are wholly uninfluenced by English ones. Hence anglophones, especially monoglot anglophones, can have some difficulty when they encounter Irish words.
Irish adopted the Latin alphabet at a time when that alphabet had not yet developed into the one we know today — some letters were yet to emerge, and some were still regarded just as variants of other letters. So Irish is written with an alphabet of only 18 letters — Irish did not use j, k, q, v, w, x, y, z. (They are used nowadays, but only in loan-words borrowed from other languages.)
This relatively small number of letters has to be used to signify a relatively large number of phonemes. Irish has 44 or 45 phonemes, depending on the dialect spoken. (Most variants of English, by comparison, have 36 or so.) So a lot of phonemes are represented by combinations of multiple letters (supplemented by a couple of accent marks). English uses letter combinations as well (-sh-, -th-), but Irish uses a lot more of them.
In the mid-twentieth century there was a spelling reform, to eliminate one of the accent marks and to simplify some of the consonant groups. But, as is often the case with spelling reforms, proper nouns such as names often continue to be spelt using the older conventions, or they are found using both older and modern conventions.
Finally, although written Irish has a single form, there are regional pronunciation variations.
So, take the name Siobhán, mentioned in the OP. The pronunciation of the central -bh- is affected by the fact that it’s bracketed by the vowels o, a; it would have a different value if bracketed by i, e. But it also has a different value depending on where you learned your Irish; in some places it has a sound that, in English, would be signified by -v-; in others the sound elides towards a -w-.
Iarfhlaith, also mentioned in the OP, is one of the names spelt using older conventions. Simplified versions of the name are also found — Iarlaith and Iarla.
