Following Thurneysen (ZCP 9, 312), OIr. fairrge, foirge (f, i̯ā) ‘ocean, sea’ (dil.ie/21164) is commonly understood as an abstract of the adjective fairsiung, foirsiung (u) ‘ample, broad, spacious’, i.e. ‘vast extent (of the ocean)’. Although this explanation is appealing semantically, phonologically it does not work. Fairsiung itself is a compound of intensifying for- + *eissiung ‘wide, vast’ < PC *eχsangu- ‘un-narrow’ < PIE privative *ég̑ʰs- + h2m̥g̑ʰú- ‘narrow’, cf. W ehang ‘wide’ (D. Wodtko, Altirische Sekundäradjektiva, 170–1). The simplex *eissiung is not attested as such in OIr., but its antonym cumung ‘narrow’ is (see https://www.academia.edu/…/Two_Continental_Celtic_Studies_t…, 108–9, for the wider word-formational background). Since fairsiung has already undergone syncope, the further addition of the abstract suffix -e regularly yields fairsinge ‘width, extent’, which is indeed attested. An additional, analogical syncope of fairsinge > *fairs(n)ge is not expected, nor is there any reason why the s should be lost. OIr. did not have a phonotactical problem with complex clusters like this with medial s arising from syncope, cf. airscél ‘famous tale’ or tairsce ‘some part of a shield’. I can see two alternative etymologies for fairrge:
1. It could be a compound of the preverb/preposition for ‘over, upon’ + a noun derived from the W1 verb srengaid ‘to pull, drag, draw’ < PIE root *√strengʰ- ‘to twist’, i.e. PrGoid. *u̯or-sreng-ii̯ā. The meaning would be similar to the one suggested above, namely ‘extent, width < *drawing wide out’, applied to the ‘extent of the ocean’. The diachronic phonology is regular in this case: the middle e would be syncopated with concomitant palatalisation and loss of the nasal, and the cluster -rsr- would be simplified to -rr- since the formation of this word belongs to a much earlier period than the one suggested by Thurneysen in his explanation of fairrge; cf. also the behaviour of the compound do·srenga ‘to draw, drag, pull’ whose prototonic stem is ·tairr(n)g-. A compound of fo-sreng- ‘draw/pull under’ would work even better formally, but the semantic development is unclear.
2. Another possibility is to connect fairrge with the topographical term Οὐεργιούος = /u̯ergiu̯os/ transmitted by Ptolemy for a part of the Atlantic south of Ireland (thus already Stokes, Urkeltischer Sprachschatz 273). This name seems to be related with OIr. ferg ‘fury, anger’ < PC *u̯ergā < PIE *u̯erHg̑eh2. Fairrge could conceivably go back to PC *u̯ergiu̯ii̯ā ‘the wild, furious one’ (less likely *uergii̯ā which should have given *fergae), referring to the less agreeable aspects of the North Atlantic. The vowel of the first syllable in fairrge would have to be explained through some analogical influence. *uergii̯os may underlie W Y Môr Werydd ‘the Irish Sea, the sea west of Britain’.
An Old Irish Poetic Formula
Probably every student of Old Irish is familiar with the poem Is acher in gaíth innocht ‘The wind is sharp tonight’ (https://www.asnc.cam.ac.uk/spokenword/i_isacher.php?d=tt). Its second line has the wonderfully poetic phrase fo·fúasna in fairrge findḟold ‘it tosses the ocean’s white hair’. It seems to have gone unnoticed that the last 4 syllables of the line constitute a veritable poetic formula of Old Irish. Is acher in gaíth innocht, which has Viking attacks as its topic, must postdate 795, and predate 850–1 because it is contained in the precisely-datable glossed Priscian manuscript from St Gall (cod. 904). Appr. 50–100 years older is the first occurrence of the formula in l. 912 (st. 228) of the Poems of Blathmac:
Is hé tuargaib tuinn do thrácht
co·mbáidi benna borrbárc;
is é tróethas anfad ngréich,
fo·cheird for fairrgi findḟéith.
‘It is he ( = God) who raises the wave to/from the strand
so that it drowns the prows of proud ships;
it is he who subdues the screech of tempests,
who puts a fair calm upon the ocean.’
The formula is used again twice in the possibly 11th century poem Oíbinn beith ar Beinn Étair ‘Delightful to be on Benn Étair’, found in Brussels, Bibl. Royale, MS 5100–4, p. 35 (ed. Reeves, Vita Columbae, 1857, 285). The second stanza is also found in Rawl. B 512, f. 126b (http://image.ox.ac.uk/show…).
Oibind beith ar Beinn Edair,
re ndul tar fairrge findḟind.
Turracc tuinde ‘na hacchaidh,
luime a caladh ‘sa himild.
Oibhind beith ar Beinn Ettair
re ttecht tar fairrgi fonngil,
beith occ iomram a curcán,
uchan sa tracht tondmir.
‘Delightful to be on Benn Étair,
before going over the white hair of the ocean/the white-haired ocean.
The wave’s onslaught against it,
the barrenness of its harbour and its border.
Delightful to be on Benn Étair,
before going over the fair-bottomed ocean,
to be rowing in a coracle,
oh!, on its wave-crazy strand.’
The formula consists of the word fairrge followed by a compound made up of find ‘fair’ + another noun or adjective, also starting with f (but not in fonngil). In Blathmac and Is acher in gaíth innocht, even the words in the opening of the line start with f. But, as can be expected, there are also differences between the instances. Fairrge can be in the accusative, followed by an adjective, or it can be a preposed genitive before a noun (in Is acher in gaíth innocht and perhaps in the first stanza of the Benn Étair poem). There is also metrical variation: whereas in Blathmac’s poem and in Is acher in gaíth innocht the formula occurs in the second line of a deibide couplet, forming the ardrinn of a rinn-ardrinn rhyme, in Oíbinn beith ar Beinn Étair it constitutes an isosyllabic rhyme in a rannaigecht metre.
These poems are clearly intertextually connected. Two scenarios are possible. Maybe ‘fairrge find-X’ was a traditional poetic formula or cliché learned by filid in their education. In that case the three instantiations could be independent of each other. Or the poems draw upon each other. In that case Blathmac as the earliest would be the inventor of the formula.
Geographically there is a connection between the three poems. Blathmac comes from the Louth-Monaghan area. In ITS Suppl. 27 (https://www.academia.edu/…/The_Language_of_the_Poems_of_Bla…) I tentatively suggested for Blathmac the possibility of an affiliation with the church of Lann Léire (Co. Louth). However he could well have been a member of the monastery in Bangor, Co. Down, which is not very far away. Is acher in gaíth innocht was in all likelihood written in Bangor. A Bangorian provenance is assumed for the manuscript in which it is contained, and the poem, with its maritime concerns, would very well suit the location of Bangor at the sea where it was particularly vulnerable to Viking attacks. Finally, while nothing is known about the provenance of Oíbinn beith ar Beinn Étair, its theme is Colum Cille and his passage to Iona, so we are still in the north-east of Ireland. Mutual knowledge of the compositions in this well-circumscribed area is easily conceivable.