Easter, 1916 by WB Years

A “terrible beauty” references Book 2 of the Iliad. Helen of Troy is described as possessing “terrible beauty” - terrible, because it is a cause for war.

The “terrible beauty” (cause of war) here is the martyrdom of his good friends at the hands of the English.

For most of the UKs existence, it included Ireland. Even after the Republic of Ireland was granted independence, the UK sought to treat the ROI as an extension of itself.

Winston Churchill offered Irish President De Valeera Northern Ireland if he committed Irish troops to WWII. De Valeera refused.

Ireland consistently sought to distance itself from the UK military.

To this day, Ireland has refused to join NATO, partially because it would obligate them to defend the UK. (The Irish are less fussy about UK fighter jets defending Irish skies from Russian interlopers).