The better question is "When did 'ye' stop being used?"
English didn't always have just one word for "you". Check out this pattern, see if you notice anything:
I - me
you - you
he - him
she - her
we - us
you - you
they - them
All of the other pronouns come in singular and plural, subject and object. Why not "you"?
It used to. Long ago, the second person singular was "thou - thee" and the second person plural was "ye - you". Over time, these were lost in Standard English as it all collapsed into just "you", first by losing the "ye/you" distinction, then by losing the "thou/you" distinction. But there are still a handful of dialects that have preserved "ye", often collapsing "you" into "ye".
It's a colloquial form of the second person plural in English (the plural "you"). I know it is at least used in southern Ireland (where I'm from) but it could be used elsewhere. We included it since we also included "y'all", "you guys" and other colloquial forms.
I'm just getting used to sibh in sentences and find its uses as polite form of 'you'
As far as I know, there is no informal/polite distinction in Irish, only singular/plural.
tú = singular you
sibh = plural you
Can anyone tell me when "sibh" is used for "you" instead of "tu" (diacritic ommited)?