Why are Celtic spellings and pronounciation so different from other languages using Latin script?
Why are Celtic spellings so different from other languages using Latin script?
Why does the presence of H change the sound of many consonants ?
How to memorize the spellings and pronunciation of languages like Scottish and Irish?
Celtic spelling, especially for Irish and Scottish Gaelic is so different from other languages using the Latin script for a few reasons. Firstly, the spelling conventions were mostly established in the early Middle Ages. The language has changed a lot since then, but what spelling reforms they’ve undergone have not been too drastic. In addition, the Latin alphabet was never a good fit for these languages. It doesn’t do a good job of representing consonant mutations, the rich vowel inventory, or the differences between slender and broad consonants. All of these are essential for proper pronunciation and grammar. The somewhat clunky system we have now is really the best anyone can manage.
The presence of the letter h after certain consonants is known as “lenition”. This is a phonological phenomenon where the pronunciation of an initial consonant changes due to its grammatical environment. All Celtic languages have similar phenomena. I recommend reading the grammar notes on Lenition for the Irish course here: https://duome.eu/tips/en/ga. I’m new to Scottish Gaelic and some differences between the two languages might exist with regards to when lenition is used, but so far they seem to be the same.
As for memorizing the spellings and pronunciations: you’ll just have to do it rote. Eventually you’ll notice patterns and rules on your own. The consonant mutations make sense if you understand the basics of phonology, but if you don’t it will be quicker to just memorize everything than learn about phonotactics.
Until that gets here, I found this to help a bit: https://en.wikipedia.org/wiki/Scottish_Gaelic_orthography. It doesn't have any sound files and requires knowledge of IPA, but it does give a detailed account of all the possible pronunciations of all the different letters and their combinations. This combined with practice may help you make sense of the pronunciation faster.
There's a super synopsis of the basic rules pronunciation rules here, from VarsaTinyArnen -- you'll have to scroll down to find it: https://forum.duolingo.com/comment/35280755/Tha-G%C3%A0idhlig-snog
The big reason is Latin script was intended for the Latin language. You can see this in English with all of its pronunciation. e.g. ‘dove’ and ‘dove’
Through Though Thought Rough
The ‘ough’ has a different sound in each of these words.
As for Celtic languages it is just a matter of time to get use to its pronunciation.
The pronunciation of all modern Gaelic languages has changed considerably since the middle ages. Old Irish had sounds that the modern languages lack, and the modern languages have sounds that Old Irish lacked. Scottish Gaelic spelling is not a perfect representation of medieval Gaelic spelling, much like how written modern English is a poor representation of medieval English. Both modern languages clearly base a lot of spelling conventions on those of their predecessors, but there are still many obvious differences. To answer your last question: not much is known about medieval Scottish Gaelic phonology. The language was not written down very often until the early modern era. Most of the Gaelic writing published in the Gàidhealtachd up until that point was in Classical Irish, a polished literary version of Middle Irish.
I was told by a Gaelic teacher years ago that Gaelic, both in Ireland and Scotland, was originally put into writing by the Latin-speaking monks who were sent there from Rome. So Gaelic spelling isn’t going to look like anything other than itself. It was the monastics’ best shot at representing the sounds of Gaelic using Latin sound values for letters, and they had to improvise when it came to lenition and other features not present in their Latin.