I did a quick search and found two other children born to William Johnstone and Anne Croane.
David, born 1845 and Anne, born 1850.
I found this on Roots Ireland. For the roots Ireland website you will have to register and pay for credits in order to see the results. You can search for free like I did but as I said you will have to pay to see any more information.
You said that the family lived in the Clonleigh/Lifford area. Maybe you could contact the parish liason in the Clonleigh parish and also the raphoe parish here on reaching out.
This is also a good website when researching people from Donegal - It is called Donegal Genealogy Resources
You should also think about contacting the local churches in both Clonleigh and Raphoe to see if they have any information.