The 1901 Census of Ballybrooney shows 3 families of Walsh.
John Walsh 50 wife Mary and children. John married Mary Duggan in 1882 and his father was Patrick, witness was Patt Sweeney, their eldest child was Patrick, their daughter Agnes in census married in 1932 Balliiskerry Church a Patrick Durcan of Killala, her father was listed as John of Ballybrooney.
Michael Walsh 56 wife Catherine and children. Michael married Catherine Sweeney I have been unable to find a marriage for this couple to confirm his fathers name. The eldest child was Patrick born in 1881, usually the eldest is called after the grandfather (not always though). If you can find the marriage it well may confirm Michael Walsh's father was Patrick also.
Its possible that Ann Walsh shown living in 1901 with son Michael was the wife of Patrick Walsh shown in the 1856 birth you have of your ancestor. The marriage record you mention in 1875 the witness would not be the father Patrick this is normally a sibling or a cousin, Patrick is listed as her father, the dates given on marriage records are not to be taken as accurate as with the census records, she would have been 20yrs old not underage, normally over 21yrs is often shown as full age.
The 3rd family are brother and sister John Walsh and Mary the ages in Census 1901/1911 vary greatly another sister Ellen Gillespie widow living with them in 1911.