How was the Irish otherworld described in early Celtic tradition?

In early Irish tradition, the otherworld was conceptualized as a realm existing either underground or as islands located in the sea. Often referred to as the "Land of the Living," "Delightful Plain," or "Land of the Young," it was depicted as a place free from sickness, aging, and death, where time functioned differently and happiness was eternal.1

This realm was also associated with the side (burial mounds), where the Tuatha Dé Danann were said to reside after being driven underground by the Milesians. In Irish mythology, the otherworld was frequently accessed by heroes through magical mists or voyages, often guided by supernatural figures like Manannán or Lugh.21

While this "delightful plain" represented a paradise for the fortunate, it was not the destination for everyone. The god Donn reigned over the dead at Tech Duinn, a location associated with the afterlife where the majority of the deceased were believed to return.1