Quick answer
Book Time Off adds Ireland’s public holidays to the calendar automatically, marked as non-working days. They are never taken off anyone’s allowance, and the list is a sensible default you can edit.
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Public holidays are automatic
The Public holidays field shows Ireland (public holidays) and is read-only. There is no region to choose and nothing to switch on · Ireland’s public holidays load automatically, appear on the calendar and stay up to date on their own.

Public holidays are never deducted
A public holiday is shown on the calendar but is never counted against anyone’s allowance. If you want to close on a day that is not a public holiday, use blocked dates instead.