It sits in pasture about forty metres north of Spa House, forming a quiet pond from which a stream runs off to the northwest, and local people have always called it simply the "spa well". The name points to something more geological than spiritual: this is a warm spring, fed by water rising from depth at a temperature noticeably above the ambient ground, and it belongs to a broader cluster of such springs in the Mallow area that drew attention long before anyone thought to classify it archaeologically.

According to research published by Brück in 1987, the Spaglen spring is the largest of the Mallow warm spring group, measuring 13.6 metres in diameter and reaching several metres in depth at its centre. That makes it a considerable feature, even if its low profile in pastureland means it rarely announces itself. Warm springs of this kind were associated during the eighteenth and nineteenth centuries with therapeutic bathing and the broader spa culture that gave towns like Mallow a fleeting fashionable reputation. The proximity of Spa House, which presumably takes its name from the spring rather than the other way around, suggests the site was once considered worth developing. More recently, works to the house and its grounds have included stone revetting of the pond, reinforcing its edges with a lining of dressed stone to stabilise the banks.