Here's the answer given in a Gaelic grammar I read a long time ago. Gaelic spelling hasn't changed greatly since the Middle Ages but the pronunciation has (like all languages). The old spelling became almost sancrosact,however; probably during the times when the language was more or less outlawed. All those extra h's are there mostly to show what letters not to pronounce. I've probably forgotten a bit but that's the gist of it. Written Tibetan has a similar pecularity.