This also initially confused me also as my ancestors were Catholic and their address on the 1851 Census was also the Neilston Manse. Neilston Manse was the residence of the Church of Scotland minister and his family. It is not really possible that that three Irish Catholic boys were residing at this residence. In an effort to understand this record I made contact with a member of RootsChat living in Neilston.
I am grateful that MurphyZ from RootsChat helped me to understand this. Here is her reply to me..
"I have gone to Barrhead Library and looked through the microfilm of the census; the first section of the census definitely is the Manse with the minister of the parish living there, 6 people at the Manse itself. Then there is clearly a thick line under it and it continues with lots of other entries, which to me mean a new address. But there are no other addresses just pages and pages of names separated by thick black lines. Joel Harkin's entry is quite a bit further down the census from the Manse."
It would seem that the brothers resided in the same area or block as the Neilston Manse (not at the Manse inself) but at a lodging house probably built to accommodate mill workers. The occupations of their neighbours included bleachers, calico print workers, power loom weavers, spinners, servants, agricultural labourers etc. The head of the house or landlady was a cowfeeder. Many of the residents in this area of the town were born in Ireland. Hope this helps...