https://en.wiktionary.org/wiki/frith

.....From Middle English frith, from Old English friþ, friþu (“peace, tranquility, security, refuge”), from Proto-Germanic *friþuz (“peace, reconciliation”), from Proto-Indo-European *priHós (“beloved, happy”). Cognate with Dutch vrede (“peace, quiet, tranquility”), German Frieden (“peace, tranquility”), Swedish frid (“peace, serenity”), Icelandic friður (“peace, tranquility”). Related to free, equivalent to free + -th (freedom/wiki/frith ..........


An Etymological Dictionary of the Gaelic Language (F)
fret, freit.
frith , frioth small, trifling ( Sh., O'R.), which M`A. says antecedes the noun, is the prep. frith or ri.
frith a sour or angry look (A.M`D.), frithearachd, peevishness, Irish frithir, peevish: *vr@.ti-; root of ri "against"?
fr?th a forest, deer forest, Irish frith, wild, mountainous place, Welsh ffridd, forest; from Middle English frið, deer park, Anglo-Saxon frið.
"In north Wales, 'ffridd' means land with no borders or land in a large field surrounded by a wall or fence. 'Coedcae' is a south Walian term referring to a wooded area."

The latter sentence is a little confusing as Coed is in North Wales as well eg coed y brenin outside Dolgellau (with coed meaning wood)
Interestingly the battle cry of the Macgregors is Ard Choille(the wooded heights)
and the irish Coillte is a semi state enterprise managing nearly 500,000 hectares of forestry.