I’m assuming that your query is about the Margaret Cowell who married John Dovaston at St Nicholas Church, Liverpool on 28 June 1846 (from your information “born Sulby” and “father James Cowell, stone mason”)
The marriage register image shows that both Mary and John signed with an X-mark. I suspect that the vicar wrote the entry as “Cowell” because that was what he thought he heard. But I believe that Mary was probably Mary Cowle, and that she was the eldest child of James Cowle and Margaret Stephen/Stevenson (who married in Ballaugh 26 Aug 1814, both signed with an X-mark, and the vicar wrote the surname as Cowel ).
Here’s my reasoning:
First, I spent some time searching for the Mary Cowell outlined in your message. I looked for other children born to the couple you suggested (James Cowell and Elizabeth Kerruish). Then I looked for the family of James and Elizabeth with those children in the 1841 census. I believe I found the family in Ballaugh, but James Cowell was an agricultural labourer --- so I realised that you’d probably “got the wrong Mary Cowell”.
Next, I looked for Mary and John Dovaston in the Liverpool censuses. Here are the ages Mary gave between 1851 and 1881:
1851 age 30
These would suggest a birth year of approx 1817.
I’ve found a John and Mary Dovaston couple in Shropshire in 1891 but I believe that it’s a different couple, so I’ve ignored that entry.
Sulby is in the parish of Lezayre. There was a baptism of a Mary Cowle in Lezayre on 13 Feb 1816 to James Cowle and Margaret Stevenson. This same couple (James Cowle and Margaret Stephens) had several more children, all baptised in Lezayre:
Jane 4 April 1819
Margaret Anne 27 March 1822
John James 20 Nov 1825
Thomas 22 Feb 1829
William Daniel 15 Jan 1833
Eleanor 5 March 1837
James Cowell 45 mason
all born IOM
Old Sulby, Lezayre
James Cowle 58 Head
Margaret Cowle 56 Wife
John Cowle 25 Son
William Cowle 18 Son
Elener Cowle 15 Daughter
JC 58 stone mason master b Lezayre
MC 56 wife b Ballaugh
JC 25 son stone mason journeyman b Lezayre
WC 18 son stone mason apprentice b Lezayre
Elener 15 dau at home b Lezayre
So this looks like the right family, Mary having left home before the 1841 census.
I wouldn’t worry about the confusion in the spelling of Cowell/Cowel/Cowle --- they often get confused.
If you’re not familiar with familysearch.org, you need to register (free). There you’ll find a treasure trove of records, many of them scanned images. If you find it tricky to navigate, just ask for help.