Ethica Terra, ‘the land of Eth,’ also called Ethica insula, ‘the isle of Eth,’ is Tiree ; the sea between Tiree and Hì [Iona] is called Ethicum pelagus. In other Latin Lives of the Saints it is called Heth, Heth regio, terra Heth. In Irish literature the name occurs, so far as I have noted, only twice, once on the Book of Ballymote (205 a 11), where the Cruithne are said to have gone from Ireland to ‘Tir-iath beyond Islay’ (i tir iath seach Íle), and again in Rawl. B. 502,* 115 a 5, where Labraid Loingsech is stated to have razed eight towers in Tiree (ort ocht turu Tiri iath), both already referred to. The poem in Rawl. is very old. The fact that Adamnan’s Eth- becomes in O[ld] Ir[ish] iath proves that the e of Eth- is the long E[arly] Celtic e (for ei), which is retained in the very oldest specimens of O.Irish, and in some Irish names in the Latin Lives of the Saints, but which by A.D. 800 had been broken to ia when it was followed by a broad vowel in the next syllable. Thus eth becomes iath, as Ceran becomes Ciaran. This, as Kuno Meyer has pointed out, is fatal to Dr. Reeves’ derivation from Old Irish ith, gen[itive] etho, corn, attractive as it is in view of Tiree’s proverbial richness in barley. In the twelfth century Reginald of Durham has Tirieth. Subsequent forms are Tiryad, 1343 ; Tereyd, 1354 ; Tyriage, 1390 ; Tyree (Fordun, c. 1385) ; Tyriage, 1494 ; Tiereig, 1496. A learned Gaelic poet of the sixteenth century writes Tír igedh (i.e. Tír-igheadh). In Gaelic now with the people of Tiree it is Tireadh; outside the island it is Tìr-idhe or Tìr-ithe; but one also hears Tìr-idheadh, with a distinct stress on Tir, which is very like the form used by the sixteenth century poet. I have also heard Tìr-iodh, or Tìr-eadh. The Norse form Tyrvist (compare Ivist, the Norse form of Uist), but -vist can have no phonetic relation to O.Ir. iath1. The modern Tiristeach, a Tiree man, must come from this Norse form. The modern Tìr-iodh may be compared with Magh-iodh or Mag-eadh, near Crieff, anglicised as Monzie; in Tìr-idhe, Tìr-ithe, the second part is the same sound as Idhe, Ithe2 the gen[itive] form of Ì, Iona. The second part of the old Gaelic Tìr-iath is the same in form as iath district, region. But the various forms, ancient and modern, cannot be reconciled with each other, and their diversity indicates that the second part is not Gaelic, possibly not even Celtic.
* Rawl.B 502 : a Collection of Pieces in Prose and Verse in the Irish language, published in facsimile with an Introduction and Indexes by Kuno Meyer ; Oxford.
1 O. Norse vist means (1) an abode, domicile ; (2) food, provisions, viands. Tyr-vist seems to point to a folk etymology of Tir-ithe as from O. and M.Ir. ithe, the act of eating, translated by vist.
2 In O.Gaelic Ie ; the dh or th of Idhe is not organic, but merely a phonetic device for separating the syllables.
Extract from The History of the Celtic Place-Names of Scotland by William J. Watson, reproduced by kind permission of Birlinn Ltd (www.birlinn.co.uk).