There is a possibility that marriages are recorded on their baptismal records however these are too to be on line -
http://registers.nli.ie/parishes/0150Ferbane is online up to 1877
The church may be able to help
If you use
https://www.irishgenealogy.ie/en/ to search for their civil birth records some are missing
I can see good possibilities for some (William, James, John, Catherine, Mary Bridget, Peter and Margaret) in Parsonstown/Birr but not the others
However for marriages and deaths you would need to know where to look or else invest in many certificates to be sure
Possibly Bridget's death aged 1 -
LINKPossibly William's death aged 21 in 1906 -
LINKHere you can see the records for Margaret's birth and probable death -
LINKStarting at home there are 16 marriages between 1911 and 1950 listed in Birr with one party named JOHNSON/JOHNSON/JOHNSTONE
You can rule out the names that you don't have e.g. Martha etc.
What is left is quite a short list and worth the investment in obtaining certificates
(€4 each from the GRO) to see if they are the correct people
unfortunately no easy way to work forward with families