If Isabella was born out of wedlock, then there is no reason it shouldn't be recorded as Neill, unless her mother chose to mention fathers name on birth record.
Most folk will recommend that you try and get as many indexes for each credit (20p) that you spend on SP. Aim for 25 for each credit.
I had a quick play with your name (see screen shot).
For 2 credits, this will give you every Isobel, Isabel, Isobella, Isabella, Isa etc from 1870 to 1880. It'll also include mis-spellings such as Neil / Neal.
You can then go through and chose which ones you think are most likely, and pay the extra 5 credits (£1) for those.
You obviously don't need to pick such a wide date range, I only did this as an example. Using 73 to 76 gives 24 results. Depends on how sure you are of year of birth.
PS If you suspect mother came from Ireland to Scotland, I'd look at births in Greenock or Glasgow first.
Dave