I have been doing research on my wife's family and she is a descendant of Isobel Dallas and Duncan McKerrell of Islay. Duncan and his children did move to Mobile Alabama sometime after 1851.
One family researcher has listed the same information that you have given that Isobel was the daughter of William Dallas and that she was born in 1803.
The only real information that I have to go on for Isobel is her age on the 1841 Islay census which is given as 32 years. Unless this is an error she may have been the daughter of John Dallas and Janet McGilveray who had a daughter Isobel born in 1808.
I am not sure if I am putting too much emphasis on this one census listing but I am intrigued that the McKerrells came to Alabama because of their relationship with the Dallas family.