Hello owen,
There are three items you require when searching for Irish ancestors.
1. Denomination.
2. Parish.
3. Townland.
You'll have to rely on Church records as civil registration in Ireland was not introduced until 1864. Non Catholic marriages were registered from 1845 onwards. When registration for BDMS started in 1864 some people didn't bother with the registration for a few years after that date. Keep your fingers crossed that church records still exist.
Did William and Janet have any children in Scotland? Did they and their children die there? It's possible that you may be able to obtain some more information about them by looking at Scottish records. Their death certs as well as their children's birth and marriage certs may contain such details.
Christopher
