Hello
There were also a few oil works around Paisley and Linwood in the 1870s and 1880s. It's possible that your family relocated for a short time between the 1871 and 1881 census if you have them in both years in Uphall. The closest to Carlile Quay is the Abercorn/Inkerman Works and you can see some information about it here:
https://www.scottishshale.co.uk/places/oil-works/oil-works-renfrewshire/I have a family of miners where all of the children were born in Airdrie, except for one 'random' birth in Ayrshire in 1858. The family had moved for a short time and I would never have known if the child hadn't been born, as they were back in Airdrie by 1861.
Who registered the birth? I've seen several births for people 'visiting' when the birth took place, and if it is a parent who registers, the 'usual address' or 'domicile' is sometimes recorded if different from where the child was born. If there isn't an indication of the parents living somewhere else, it might be more likely to mean that the family were themselves resident at the address the birth took place.
You can also search the 1875 valuation rolls for Paisley via ScotlandsPeople. If you limit your search to 'Burgh of Paisley', choose 'Tenants and Occupiers' for Status and add 'Carlile' in the last box titled 'place', it brings up all the records of people living at Carlile Place (there are no records for Carlile Quay). There are 22 names, none are McChrystals, but you might recognise an in-law or other family name.
If you replace 'Carlile' with 'New Sneddon', there are 200+ results, again no McChrystals
There are some McChrystals named in the Paisley Poor Relief Records, and Renfrewshire Libraries have created an index here, which you can follow up with them if any of the information matches your relations:
The Paisley Poor Records Index is here:
https://libcat.renfrewshire.gov.uk/iguana/www.main.cls?surl=PoorLaw#:~:text=What%20are%20the%20Paisley%20Poor,due%20to%20age%20or%20incapacity.
Best wishes
Rockford
