McPhail_7 on Family Tree Circles

Hi fellow family tree circle investigators
I have been researching my family tree now for about 18th months. I use ancestry,scotlandspeople,ancestry Ireland. I have been following leads from ancestry trees and find some are not correct. If you are going to follow the line do it right. It’s very frustrating at times but when you stumble over a little gem that pulls family together it’s awesome. I’m sure it becomes like a drug.
I’m searching my father’s mother side the McPhail’s. Her father Donald McPhail(jnr)1837)16 months old, arrived 1838 on the Boyne in NSW Australia with his parents Donald McPhail(Snr)1809 & Mary Ranken 1807. It’s very tricky when all the brothers of these families names their children the same name so finding the correct one is difficult sometime. On the shipping records for Donald McPhail (Snr) & Mary Ranken , is states that Donald (Snr) father is Alexander McPhail 1790 from Mull. I have Donald McPhails death certificate and it states that Alexander McPhail is his father and his mother was Ann Rankin.
I cannot find a marriage for Alexander & Ann. I guess I’m trying to find where else to follow this trail. Just putting it out there for any help or where to go next.
Thanks in advance
Sue