Apparently, some people use "y'all" as a singular address, and it doesn't necessarily indicate a plural "you", so it's essentially a plural with a singular aspect, whereas "you" is a singular with a plural aspect.
sibh and bhur are never used for the singular "you/your".
bhur = your (plural) so when it's the plural form it causes the noun following it to have a prefix. When the noun starts with a vowel the prefix used is n with a hyphen, hence ár n-arán, bhur n-arán, a n-arán for our/your/their bread.
When the noun starts with a consonant the prefix used depends on the consonant. Some examples: ár mbuachaillí, ár gcailíní, ár bhfeachtas, ár nglasraí, ár bpiobaire, ár dteanga.
It looks to me as if the quality of the prefix depends on the articulatory traits of the following consonant (the initialconsonant of the determined noun) whether it is labial, dental, velar, etc... What part of the mouth or tongue or lips is involved. Am I right?