It took some time for Fitzgerald to become the standard surname.
I retrospectively name them all Fitzgerald from the start
(except for the line that became Fitzmaurice).
The old flag of Ireland,
the Cross of St.Patrick
(red diagonal cross on white background),
may come from the arms of Fitzgerald, Earl of Kildare.
See origin.
It now (since 1801) forms part of the Union Jack.
Place names in Dublin derived from the Fitzmaurice branch: