Where does “acra-” come from?

acra- (Irish) comes from Ancient Greek ἄκρος, from Ancient Greek ὦμος, from Proto-Indo-European h₃émōs — shoulder.

acra- (Irish): acro-

Definitions

Step	Language	Word	Meaning
1	Ancient Greek	ἄκρος	At the edge, extreme, beginning, end: outermost;...
2	Ancient Greek	ὦμος	the shoulder with the upper arm
3	Proto-Indo-European	h₃émōs	shoulder