The AHD claims that *h₂oi̯u- was a variant of *h₂i̯eu- "youth, vigor" altho I haven't seen anyone else make this claim. *h₂i̯eu- is the source of Old Irish óac "young" which combined with the abstract noun-making suffix -lach to become Irish Gaelic óglách "youth, servant, warrior".
Óglách is the second part of the Irish Gaelic gallóglách, the first part being from gall "foreigner, stranger" - borrowed from Latin Gallus "Gaul" (according to MacBain's). A gallowglass was a mercenary class in Scotland and Ireland, but its etymological meaning is apparently "foreign warrior, possibly Gaulish".
The mercilesse Macdonwald
… from the Westerne Isles
Of Kernes and Gallowgrosses is supply'd
- Macbeth I, ii
(A kerne is an Irish foot-soldier, from Irish Gaelic ceithern.)