Most of the Gaels who enter the Otherworld settle in Tír Tairnigire, the Land of Promise. Originally they settled in the highlands north of the River Donu, but in recent centuries newer Gaels formed new communities in the central forests and the southern plains.
Time passes slower in the Otherworld than it does in the mortal world. The people who live in Tír Tairnigire do not age. When the Aos Sí and the Tuatha Dé venture east from Tír na nÓg, it is usually to Tír Tairnigire. The ancient ones always arrive by boat, preferring to dock in the town established by their own Manannán mac Lir.
Gaels in northern and central Tír Tairnigire trade rumors of a secret passage into Tír na nÓg somewhere in the Yellow Mountains, but no one has ever admitted to finding it.
The sun rises and sets on Tír na mBeo just as it does in every other part of the Otherworld. Yet the island is only place in the Otherworld where people age, likely the reason it's known as the Land of the Living.
Children eventually reach middle age--at a much slower rate than they would in the mortal world. Elderly people living here age backwards until they reach middle age. This slowing doesn't just affect its people. The island's unusual properties affect crops, animals and most flora as well. Food made there takes longer to spoil, even if its fresh taste fades.
Families with babies and young children taken too early from the mortal world live in Tír na mBeo for a time, if only until their child becomes an adult. People can come and go from the island as they please, but their aging ceases until they return.
Tír fo Thuinn is often referred to as "Land under the Waves" by mortals. They're half right. The area was much larger centuries ago. From the north came a towering wall of water that buried half of the land, dredged new channels and displaced large amounts of earth.
It is a hearty, chilly place where the sturdy and self-sufficient thrive. Countless members of the Fianna chose Tír fo Thuinn as their otherworldly home after their passing from the mortal world, serving as the unofficial wardens of the land.
Several high commanders--many of whom are ancient descendents of the Tuatha Dé, Aos Sí--have fallen under a curse of late that risks the safety of all who live here.
Whereas much of Tír fo Thuinn fell under the Great Wave, Tech Duinn grew. The water separated two villages from Tír fo Thuinn, and the force of the waves moved mountains and hills until the villages shared the same land as the master of the dead, Donn.
The locals on both sides of the new channel adopted the name of Donn's stronghold, Tech Duinn, as the name for the reformed island.
The cause of the Great Wave was lost to time.