Emanuel Appel Depends on which Celtic language you're referring to, mate. For the majority of their lives (over a thousand years), pretty much all of the Celtic languages were simply spoken, and there were no records. As with many Western-Europe countries, the majority of the people who could write were clergy, and they wrote almost exclusively in Latin. The earliest written version of Gaelic, as far as I'm aware, is in a manuscript called "The Book of Deer" (named after a place in Scotland, not the animal), which was written in Latin but had annotations in the margins in Gaelic about what pretty much amounted to very worldly gossip indeed. It's also significant because it is very rare in early manuscripts, which almost exclusively chronicled the lives and habits of the nobility, whereas this one talks about regular common people and even includes popular songs (such as one about a man who's renowned for having a huge and unattractive privates !)
The development and standardisation of writing systems was fairly hampered by the fact that education in the Celtic lands has almost never been in the native Celtic languages. In Cornwall, Wales, Ireland, Scotland and the Isle of Man, education was standardised throughout Britain as being in English. Even today, there are people in most of these countries who can speak the language fluently, but not read or write it, because they've never been taught. Cornish is pretty much extinct, sadly, though there are small numbers determined to revive it; Manx never had a writtwen version until an English clergyman devised a writing style for it -- in the late 19th Century, I believe -- based on what was then the spelling style of Scottish Gaelic, because the two are fairly similar. Wales is doing the best of these languages, because of a very nationalistic sentiment in that part of Britain (some say spurred on mightily by resentment towards English folk who for many years have been buying up property in Wales as holiday homes, driving up the price of real estate and displacing the Welsh themselves).
Irish and Gaelic were pretty much the same language until about 1500 years ago, when they began to diverge, although keeping many similarities because of the constant flow of population between the two in both directions.
Other Celtic Australian events 897much unmarked snippage ahead, responding only to the occasional bit that strikes me FWIW, this might have been the case in Scotland (though I doubt...
In Ireland, study of the language at school became compulsory in the mid-20th Century, and the spelling of Irish was systemised and (supposedly) simplified in 1957. I can't answer for it myself, since I've never studied Irish, but most Irish speakers I know think the spelling reforms were a disaster for the language.
Similarly, Gaelic was revised by a bunch of academics in Scotland in the mid 1970s, and adopted for education use and official publications in the early 1980s. The thrust of the changes were very different from the changes in Irish, which pushes these two languages further apart. Those Gaelic speakers I know who have a linguistic bent can usually roughly comprehend written Irish when they see it, but not when it's spoken. And without going into a lot of grammatical explanation, these changes that were made by this revision body that would supposedly make the language more accessible to learners, I think are almost universally woeful. They make no more sense to me than the dropping of hyphens in modern English, which leads to some absurd quandaries (my own favourite so far is a newspaper article about my own Gaelic singing group: at the time we had roughly twenty members -- twenty-odd, in fact -- and the article described us as "twenty odd singers". !! ;-) ) But as far as the Gaelic changes ... I don't know a single Gaelic speaker, here in Australia, in North America, or in Scotland, who likes them or agrees with them. They mostly use them because they're required to for their work, but they don't like them.
Have I gone anywhere near answering your question ? :-)
Celtic languages fall into two main groups: Breton, Welsh, Cornish and Galician form one group, with broad similarities in their languages and orthography, and Irish, Manx and Gaelic form the other group, with their own similarities. There are things linguists can point to that demonstrate these two groups had common roots, probably several thousand years back, but they're very, very different languages nowadays and mutually unintelligible.
As for the spelling of the Irish-Scottish-Manx group: Well, who decided that "mh" or "bh" would make a "v" sound, I have no idea; but then, I don't know who decided that "ch" would make the "tj" sound it does in the English word "church".
One thing I *can* tell you: Gaelic spelling is much, much, much more consistent, phonetic and logical than any dialect of English I've ever encountered. And I'm not pulling your leg on that one.
