Fortresses or castles of glass appear a number of times in Welsh and Irish mythology. One was a Fomorian fort identified with the island of Toraigh, off the coast of County Donegal, a second was identified as Ynys Wdrin, the Welsh name for Glastonbury, and the remaining examples were reported in a number of imrama or voyaging tales, including that of St Brendan.
Modern Ranger reconnaissance and expeditions have found that the part of the Otherworld reality accessible from, or approximating to, the locations of both Toraigh and Glastonbury have a high incidence of quartz crystal. The pattern of large crystals sticking through the earth and the fractured remains of pieces that have either collapsed naturally or been removed by force suggest that these locations once had structures that could have been interpreted as glass forts.
A third incidence of these crystal structures is known, christened Caer Wydyr by the Welsh Rangers that identified it, and it is in a location approximately halfway between the coast of Pembrokeshire and the coast of County Wexford. It sits on the highest point on a lightly undulating plain with enough space between individual giant crystals to take shelter from inclement weather. There is a ring of ash trees, known as the Nine Queens, surrounding the fortress. While none of them is older than four hundred years, which is typical lifespan of this group but is somewhat older than the typical ash tree, conversation with their spirit forms suggests that there has been a stand of ash there for a very long time with replacement Queens being nursed to adulthood only as the eldest fades.
Other such structures may still exist but have yet to be identified. For example, some speculate that the crystal pillar mentioned in Navigatio sancti Brendani abbatis (‘Voyage of St Brendan the Abbot’) may relate to a similar structure but others contend that it may have been an iceberg.