Sue
Not all post 1864 births are in family search.
There are also omissions from date ranges which seem to be covered.
For example my great grandmother's 1873 birth in Wexford Town is not listed. However, her siblings, born in the same place in 1864, 1866, 1868 & 1871 are listed and her final sibling born 1875 in Cork is also listed.
I have noticed similar anomalies with other births.
But, hey, it's still about the best free site for Irish genealogy!
You can always order the civil registration birth record from the GRO as you know the birth date now.
Rachel