Kilbarchan and Newtyle are some distance apart, and Stewart is a very common surname. As the census says your Margaret Stewart was born in Kilbarchan you cannot assume that the one born in Newtyle is yours, merely because her date of birth roughly matches yours. In fact the chances are that she is a completely different person.
McLaren is also a fairly common name and the IGI lists no less than three marriages of a John Stewart/Stuart to a Jane/Jean McLaren/McLaron; one in 1743, one in 1834 in Paisley and one in Clackmannan in 1850. There could easily be a fourth couple whose marriage record has not survived.
Using the free index search on
www.scotlandspeople.gov.uk I see that a Margaret Meikle or Stewart, aged 90, died in Kilbarchan in 1921. If you go to that site, you can download an image of her death certificate, which will tell you the names of her parents. This will cost you about a pound, though you have to buy six pounds' worth of credits. Do this before following up any more leads on John Stewart and Jane/Jean McLaren, because if these are not in fact the parents of your Margaret, you will have wasted time and energy on unrelated lines.
There is one Margaret Stewart of roughly the right age in the 1841 census in Kilbarchan, at Church Street (SCT1841/568/8/10)
Martha Hill/F/75/woolen [sic] winder/born Renfrewshire
Margaret Stewart/F/8/born Renfrewshire
This age is consistent with Margaret Meikle's age (48) in the 1881 census.
I used the IGI to find the children born in Newtyle, then at
http://freecen.rootsweb.com/cgi/search.pl I found the Newtyle family at Newbigging, Newtyle (SCT1841/314/1/8)
John Stewart/M/65/pendicler/not born in Angus
Jane Stewart/F/35/born in Angus
Peter Stewart/M/15/ag lab/born in Angus
Janet Stewart/F/12/born in Angus
May Stewart/F/8/born in Angus
Alexander Stewart/M/5/born in Angus
John Stewart/M/5/born in Angus
Fanny Stewart/F/1/born in Angus
It looks as if May must be Margaret, though her age is a year out.
I see that the entry in the IGI for the 1834 birth of Margaret, daughter of John Stewart and Jane Mclaren, is 'submitted' rather than 'extracted'. This makes it doubly important, if this is indeed your Margaret, to track down and check the original document from which the information was taken, to make sure that you are happy that it is valid information. (It is always highly desirable to follow up any secondary information, such as the IGI or SP, by looking at the original document for the primary source.)
Good luck!