The chapel sits to the east-south-east of Ballindoon Church in County Galway, oriented on an ENE to WSW axis, measuring just 7.4 metres in length and 5.2 metres wide externally. Inside, a graveslab rests on the floor, and to the north-east lies a small unenclosed graveyard, one that appears on both the first and second editions of the Ordnance Survey six-inch map, the second of which dates to 1897. The graveyard may be a cillin burial ground, a term referring to informal or unconsecrated burial sites often used for unbaptised infants or others excluded from consecrated ground.

The chapel was built in the eighteenth century by the Geoghegan family, who were also responsible for a mansion in the immediate vicinity. It was dedicated to St Flannan, an early Irish saint associated with the Diocese of Killaloe, though his veneration evidently extended into the west of Connacht. The building itself is plain in character: a simple doorway in the west wall, a possible second doorway in the south wall that appears to have been blocked at some point, and flat-headed windows in the east, north, and south walls. That combination of unadorned openings and modest proportions gives the structure the feel of something built quickly and without great resources, a private family chapel rather than a public act of devotion or patronage.