Frances and Jean, thank youi for your continued interest.
I have checked County Down marriages on the LDS , it was a good suggestion but no obvious registration.
I have not been able to track a definite place of birth for Sarah. County Down is shown on the Convict Indent as her place of birth and a County Down Museum claims her as being born there although I believe that this comes from the Indent detail.
I have found a possible birth place on the LDS, Magheralin is shown as the birthplace of a Sarah Mullan-father Charles Mullan.
I have not been able to confirm that this is the Sarah that I am searching for.
The census link to the Quayles' residence for John Blayney could well be a correct link, I will search the God Parents of Andrew and Jane's children.