Tobernaglas, in the townland of Glaspatrick in County Mayo, is a holy well, and its name carries the quiet layering that characterises so many of these sites across Ireland. "Tobar" is the Irish word for well, and "glas" can mean green, grey-green, or a stream, though it also echoes in the second half of the townland name, Glaspatrick, which most likely honours Saint Patrick and the particular shade of the landscape he is said to have moved through.

Holy wells occupy a genuinely unusual place in Irish religious and folk practice. They predate Christianity in many cases, functioning as sites of veneration connected to local deities or spirits of the land, and were subsequently absorbed into Christian devotion, often rededicated to saints. The practice of "pattern" days, from the Irish "patrun" meaning patron, saw communities gather at such wells on a saint's feast day to pray, walk a prescribed circuit known as the rounds, and sometimes leave votive offerings: rags tied to nearby branches, coins pressed into bark or stone. Mayo has a considerable number of surviving holy wells, reflecting both the density of early Christian activity in the west of Ireland and the persistence of local memory in keeping these places known across generations. Glaspatrick itself, as a placename, belongs to a scattered group of Irish townlands that embed the apostle's name directly into the landscape, suggesting an early and deliberate association with Patrician tradition in this part of the county.