fix gaelic plural rules

[ChangeLog][Linguist][Important Behavior Changes] The Gaelic plural
rules now know four forms. To keep existing translations intact, set
their language to Irish.

Task-number: QTBUG-40566
Change-Id: Ia3c315bb4d9bda8fac57b39c9b1a5653a1a44260
Reviewed-by: Jan Arve Sæther <jan-arve.saether@digia.com>
Reviewed-by: hjk <hjk121@nokiamail.com>