Doesn't 'rudeigin' break the golden rule of Gaelic spelling (broad with broad and slender with slender)? I have been relying very heavily on this rule (apart from days of the week which only break it because of unnecessary modernisation). Does this mean it's actually more of a somewhat tarnished silver rule than genuine gold or is this a very very rare exception (just how rare are exceptions like this). Please don't tell me this is really like a flaky English spelling "rule"
Yes, it does break the rule, and for similar reasons to the days of the week. Rudeigin (and others ending in the indefinite marker -eigin) used to be written as two words - rud eigin. There are very few exceptions to the rule and they tend to be in compound words that used to be written separately. Thankfully they are quite common words (airson - for - (formerly air son) and esan - he - (formerly e-san) are examples) and tend to fall into recognisable groups.
If you want to go down a rabbit hole, the SQA Gaelic Orthographic Conventions explains these exceptions (though not all at the one place) https://www.sqa.org.uk/sqa/files_ccc/SQA-Gaelic_Orthographic_Conventions-En-e.pdf . See you on the other side, hopefully in one piece, but that's not guaranteed.
Aha! Thank you! I understood about the days of the week but now I understand rudeigin. Plus you give a bonus explaination of -eigin, which of course comes up with cuideigin. I'd seen the similarity and was wondering if this was a THING (which it clearly is). Remarkably I never spotted airson as a rule breaker, but maybe because I learnt it before I knew the rule. I'll brave the SQA explainations if I have to self-isolate :)