"íocann do n-athair an táille" - is this a rule-breaker, unnatural, ... ?
Do is 'your' singular.
N- is used for plural.
'Bhur n-athair' would be the father of your sister and yourself: plural you. (Which the context does not exclude)
Oh nuts!! d'athair. I forgot double vowel sounds are a no no . :P
In some places, it would even turn to the unvoiced 't': t'athair.