The “Celtic Otherworld” stories are inspired by Celtic, Irish, Scandinavian and Norse legend and are set more or less contemporaneously usually with some connection to Irish or English places.
Contemporary travel to Otherworlds mentioned in Celtic Myth and Legend. Meet the Tuath Dé, Sióg, Aés Sidhe, Elves and Faerie. Also set in Limerick, Ireland and Wychavon, England.
Unlike Greek and Roman myth, there are many Celtic ‘Otherworlds’ that appear to be magical and often inhabited by the Fair Folk (Fay, Fairy, Sióg) or sometimes the Tuath Dé (later called Tuatha De Danann). They are not realms of the dead.
Manannán Mac Lir led the Tuath Dé away to the Otherworld over 2,500 years ago. Except for them it’s been more like 600 due to the time-slip. Portals are often at Court Graves, Raths and other ancient Irish sites. Today Tuath Dé culture is a crazy mix of Mediaeval to 20th Century. Most involve Alice, also called Eilis, who lived in rural Co. Limerick till aged thirteen.