Hi, took another look at this, I think you have the spelling of the place name wrong, think it was 'Mullaghagay' not Mullageagh and was a small Clachan in the townland of Three Trees, Muff, a Joseph Weir is recorded as farming there in the Griffith Valuation of 1857,(place name visible in map, not record) there is another Joseph weir recorded in the townland of Aught, Muff, I think that is the Joseph from the gravestone in my previous post, name is not common in area so both Joseph's are probably related
In the 1901 Census there is a Robert Joseph weir (45) unmarried farmer living at Three Trees, Muff a possible brother of your Rebecca, also a witness at the marriage?