Well, you have to understand the background. The GRO didn't start adding the mother's maiden name into indexes until 1929, but the records on irishgenealogy.ie have had the name added in manually when they digitised them. The records on FMP and the other sites are the indexes which the Mormons copied in the 1950s and have now turned into databases. Every year irishgenealogy will add from now on will have the mother's name but until then, there's not much you can do. When we get 1926 (in 2026 or hopefully sooner
), you'll be able to see everyone born by then.
You can also go about it the old-fashioned expensive way. Make a list from the FMP indexes of everyone born with that surname in the area during those years. Cross out any names you already have in the family. Work your way through ordering them using names that are in the family in earlier generations first. That's how we did it before all these online indexes (I sound about 100 years old!)