John Koch has recently posted a paper: Before the Branches: towards a new
understanding of (Late) Proto-Indo-European and Copper-to-Bronze Age
Europe. In this paper he explains why the native words for arrows and bows
are not found in Old-Irish but are derived from German and Latin:

He discusses that combat in Ireland and Britain changed from an earlier use
of bow and arrow to the use of prestige weapons due to the way that battles
and combats were more of an immediate in your face type of struggle rather
than one done from a distance. He suggests this is why such weapons seem to
disappear from the archaeological record in the Continental/Insular Bronze
and early Iron Ages.

"Arrows and archery wrist guards were essential components of the Beaker
package in all regions. After the Early Bronze Age, there is little trace
of archery in later prehistoric Britain, Ireland, or NW France. It is thus
not surprising that later borrowings predominate in the attested Insular
Celtic languages: MIr. boga and Welsh bwa from Old Norse, OIr. saiget
‘arrow’, Welsh saeth from Latin. Breton gwarek ‘bow’ is a deverbative

He postulates (from sources like Rhŷs, J. 1891–94 ‘The Celts and the other
Aryans of the P and Q groups’, Transactions of the Philological Society,
1891–94, 104–31.):

"§5. the common Palaeohispanic name Arquius and the regional survival of
archery from the Beaker Copper Age

‘The name Arquius probably meant one who had to do with a bow, that is to
say, an archer, and was derived from a Celtic word cognate with Latin
arquus and arcus, a bow or arch’ (1891–4, 106). ‘We have probably a trace
of the word [for bow] in Welsh arffed, “the lap or abdomen”, cf. German
Schambug’ (Ib., FN 7)."

The original words seem to have been lost along with the reporting of bows
and arrows being used in the heroic tales.

The entire presentation (slides only) is available from Koch's academia web

Searles O'Dubhain