Joseph Mulholland

Could anyone help me with my great grandfather Joseph Mulholland so i know he was born in buncrana and found his birth record in 1906 and i know it’s definitely him as he’s recorded on census with siblings that all match the names of his siblings, his father is recorded as william but the only thing with this is that on joseph’s marriage record in 1941 he has his father down as john now it’s possible maybe he was just unsure as his father did die before he was born but i’m sure his family told him his father was William i’m not sure how he would get the wrong name, so i’m just wondering is is maybe possible this is an uncle who was like a father to him or something like that, i can’t confirm this as for his father “William” the only info i can find on him is his death record and marriage record to mary murray joseph’s mother, but i can’t find any birth record for william or i can’t find if he had any siblings or anything like that and same for his wife mary i know her father was michael murray and her mother Eliza but dont know her mothers maiden name just know she was born in co tyrone, im kinda hitting a brick wall with this side of the family so just wondering if anyone could help, thanks