Dún Seachlainn "the fort of Seachlann" < Domhnach Seachlainn "the church of Seachlann"
< Old/Middle Irish Domnach Sechnaill (Modern Irish Domhnach Seachnaill) "the church of Seachnall"

Seachlann — personal name (metathesized form of Seachnall (< Secundinus))

This Irish placename has undergone a significant transformation over the centuries. The earliest surviving reference we have is from Annála Uladh [The Annals of Ulster] under the year 801, abbas Domnaig Sechnaill [“the abbot of Domnach Sechnaill”]. The word domnach is an early borrowing from Latin dominicum and is found in the names of churches founded in the earliest Christian period in Ireland.

The second part of the Middle Irish placename, Sechnall, is another loan from Latin. Secundinus was the name of one of the bishops who spread Christianity in Ireland in the 5th century. The historical forms (from Latin ecclesiastical records) show that the genitive form Seachnaill was metathesized to Seachlainn by the end of the 12th century, e.g. ‘Douenhagsachlin’ (1187), ‘Dounaghsachlayne’ (1248). The same change is attested in the related Middle Irish personal name Mael Sechnaill which became *Maol Seachlainn by the early modern Irish period.

The word Domhnach had been replaced by Dún at the beginning of the placename by the end of the 14th century, e.g. ‘Dunshaglyn’ (1374), ‘Donshaghlyn’ (1437). This change is also reflected in Irish-language manuscripts, e.g. ‘inghean an giúisdís Díolún ó Dhún Seachloinn’ in RIA MS 23 F 12 (*c.*1700). The following account of the name used by local Irish-speakers in 1836 is further evidence of the fact that this change had become established in the Irish form of the name:

Dunshaughlin is always in Irish called Dún Seachlain (pron. Shoghlin) (or Duin S.——) for which they can assign no meaning — never Domhnach S ’ ... ‘In the townland of Bonestown [parish of Dunshaughlin] there is a holy well called Tobar Saint [sic] Seachlain.’
[—Ordnance Survey Letters (Co. Meath) 263–4]