A DESCRIPTION OF THE PROBLEM :
In java 8 CalendarData_en_IE had firstDayOfWeek as 2 (Monday).
But at least from java 11 it is defined as 1 (Sunday) in sun.util.resources.cldr.CalendarData.

I am pretty sure that java 8 had it the right way, with Monday as the first day of the week in Ireland.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
open sun.util.resources.cldr.CalendarData.java and find "IE" in the group after "1:".

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
"IE" is after "2:" so Monday is the first day of the week.
ACTUAL -
Sunday is the first day of the week in the en_IE locale.

---------- BEGIN SOURCE ----------
import java.util.Locale;
import java.time.temporal.*;
var locale = new Locale("en","IE");
var firstDayOfWeek = WeekFields.of(locale).getFirstDayOfWeek();


---------- END SOURCE ----------

CUSTOMER SUBMITTED WORKAROUND :
don't trust methods for getting the first day of the week with Irish locales.

FREQUENCY : always