The small island of Eilean Trostain may contain a commemoration of a saint, though other explanations for the name are possible. If it does contain a saint’s name, it is most likely to reflect the saint best known in eastern Scotland as St Drostan, who is most famously associated with the early medieval monastery of Deer (Mackinlay 1914, 214-19; MacQuarrie with Butter, 353-4; Clancy 2008, 382-5; DoSH: Drostan). He founded this monastery, and it may be that he is the Drostán Derthaige whose death at Ardbraccan Co. Meath is commemorated in the Irish Annals of Ulster (deriving from an earlier Iona Chronicle) for AD 719 (AU 719.2; for the argument, see Clancy 2008, 382-4). It was in a 9th century gospel-book of the monastery of Deer that we have written our earliest securely Scottish Gaelic, in the form of property records written on blank pages and in the margins (Forsyth 2008). One of these tells of the founding of Deer, and depicts Drostan, probably anachronistically, as a disciple of Columba’s. Drostan’s cult in Scotland was fairly widespread, especially in the east. The most detailed account of this saint, under his various names and feast-days, is now that by Oisín Plumb, in his 2016 PhD thesis in the University of Edinburgh (Plumb 2016, 183-98).
Trostan is a known variation of this saint’s name (Mackinlay 2014, 217-18). Perhaps the most relevant comparison to the name of Eilean Trostain is to be found in Halkirk parish, in Caithness, where there was a chapel and a well dedicated to the saint under this form, as St Trostan’s Chapel, St Trostan’s Well. The saint’s cult is known elsewhere in the region, for instance, as St Drostan’s Kirk in Canisbay. It is likely, however, that his cult is not early medieval here, but rather reflects the patronage of incoming lords from Moray in north-east Scotland in the late 12th and early 13th century (Clancy 2008, 384; Clancy 2010, 389-91). Nonetheless, Eilean Trostain may at a push reflect some extension of that later medieval north-eastern cult into the western isles.
One of the earliest potential references to Drostan is a poem which cites him as Troscán, one of the seven sons of Oengus, all seemingly individuals who worked in both Scotland and Ireland (Ó Cróinín 1981, 104-8, 112-14; Ó Riain 1985, §209, 701). Though not without its problems as a source (this poem has again been most thoroughly interrogated by Oisín Plumb, 2016, ch. 6), it is worth noting that another saint who appears as his brother here is St Torannán, otherwise known as Mo Thairéin (see Plumb 2016, 200-13), who likely lies behind the name of Taransay off Harris (Mackinlay 1914, 106-7). If that is so, it would place a cult of St Torannán in the Hebrides early enough for him to be incorporated in an Old Norse name by Scandinavian settlers (there is also a Paible on Taransay). If Torannán’s cult in the Western Isles pre-dates the Scandinavian settlement, as it would seem to do, then perhaps we might imagine the cult of Drostan, as Trostan, might also do. This is all, however, fairly tenuous, and the presence of any person, still less St Drostan, is not guaranteed to be there in the name of Eilean Trostain.