"Cia mheud bus?"
Translation:How many buses?
Logically, if you are asking the question, you do not know how many there are, so you do not know if it should be singular or plural. The convention (and it is only a convention) is that we use the plural in English (even if it turns out there is only one) and the singular in Gaelic (even if it turns out there are 76).
Historically, there may be deeper reasons that I don't know about.
Just different spellings. According to something one of the mods wrote 7 months ago, the plan is to switch over to cia, presumably because it is the standard in GOC (Gaelic Orthographic Convention). Some people seem to think one is more common and some the other. Clearly both are in common use so I hope they continue to accept both. I cannot find the origin of the phrase so I cannot say what would be the more logical spelling. I even looked in an Irish dictionary, in case it told me but all it told me was it was spelt cá or cé. Very helpful.
So I then found the related word in Welsh (where they like using a p instead of a c). The big dictionary (GPC) said it was pa or pe. It said it was pi in early Breton and pe in late Breton, and that it is related to pwy (English who, Gaelic cò) and PIE *ku̯ei. Wiktionary says of pwy 'From Old Welsh pui, from Proto-Celtic *kʷēs, (compare Breton piv, Cornish piw, Old Irish cía), from Proto-Indo-European *kʷis.
The Latin is qui(s) but quo is effectively the dative/locative/allocative of the same word, and I suspect this is the key. The original PIE would have had different endings, depending on the the context and hence the case, and I think what we are now seeing is these endings being used fairly randomnly in modern languages.