Saint Brigid's was founded as a small Cistercian house in the twelfth or thirteenth century, and in 1224 became a cell of Knockmoy Abbey, the larger Cistercian foundation near Tuam in County Galway. The architecture is modest - a small nave, a chancel, the usual liturgical fittings of piscina and sedilia, lancet windows in ogee and cusped heads. There are carved heads of unknown subjects worked into the stonework. The abbey sits near the south coast of the island, a short walk from the modern post office, in ground that has been continuously sacred for at least eight hundred years.

What sets Clare Island Abbey apart from any number of similar small abbeys around Ireland is the survival of its medieval wall and ceiling paintings - one of the most important collections of late medieval secular and mythological imagery in the country. The figures include dragons, a cockerel, stags, men on foot and mounted, a harper, birds and trees. Cistercian houses were not supposed to allow this kind of imagery; the order had spent its early centuries deliberately stripping religious art down to plaster-white austerity. Whoever painted these images here, on a remote western island, was either operating outside the normal supervision of the order or working under a patron who simply did not care what Citeaux thought. The paintings have been the subject of careful conservation work in recent decades.

The abbey contains numerous tombs of the local ruling family, the O Maille - O'Malley. The most prominent is a canopied tomb on the north wall, and tradition holds that this is the burial place of Grainne Ni Mhaille, Grace O'Malley, the famous pirate queen who died around 1603. The same tradition has her baptized, married, and finally interred here, the centre of her life on the island where she kept one of her three strongholds (Granuaile's Castle stands a few miles north on the east coast). Whether the tomb actually contains her remains has not been definitively proven, but the canopy, the inscription O S O'Mhaille (the O'Malley motto, 'Powerful by Land and Sea'), and the consistency of the local tradition give the claim weight.

The abbey was probably dissolved in the late sixteenth century, swept up in the long process by which the English crown took apart Irish monastic institutions. But Clare Island was remote and difficult to reach, and the buildings did not become a quarry the way some mainland abbeys did. Carmelite friars, dispossessed from their own foundations, took refuge here at various points - an unofficial continuation of religious life on a site the dissolution paperwork had nominally ended. The dragons on the ceiling kept their colours through all of this, and through the next four hundred years of weather and wear.