Tìr in Gaelic means "land". Jovincon doesn't appear in the Gaelic dictionary. The Gaelic names for the Otherworld are either Tír na nÓg (Land of the Young) or Tír na hÓige (Land of Youth).
The otherworld is a place where one goes after death in Celtic mythology. They usually cross the sea to get to the other side. So, dying is actually a dangerous journey among the deathly waves covering the cursed depths.
The story of Oisin
Long ago, people in Ireland believed that there was a beautiful land
in the western sea called Tír na nÓg - The land of the young. It was a
place where the trees were always green, the flowers were always in
bloom and men and women never grew old. This is the story of how
Oisín, the son of Fionn MacCumhail leader of the Fianna, came to go to
Tír na n-óg.
One morning the Fianna were hunting deer on the shores of Lough Leane
in County Kerry. As they rested on a hilltop, a beautiful girl came
riding towards them on a snow white horse. She was dressed like a
princess and her long golden hair hung to her waist.
As she drew near, Fionn called out “What is your name and what land
have you come from?” – “I am Niamh of the Golden Hair and my father is
King of Tír na nÓg. I have heard of a great warrior named Oisín. I
have to come to find him and ask him to return with me to the Land of
Fionn was sad, for he feared that if Oisín went with Niamh, he would
never see him again. But it was too late, Oisín was already in love
with the princess. He accepted Niamh’s invitation and waving goodbye
to Fionn and his friends, he jumped onto the horse behind Niamh. Away
they galloped into the morning mist.
Over the land and the sea the fairy horse ran, moving as swiftly as a
shadow. At last they reached the golden shores of Tír na nÓg. The king
and queen welcomed Oisín and held a great feast in his honour. It was
a magical land. Oisín hunted and feasted and at night he told stories
of Fionn and the Fianna and of their lives in Ireland. Oisín had never
felt as happy as he did with Niamh and before long they were married.
Oisín lived in Tír na nÓg for three hundred years, but being so happy,
it only seemed like three. Then a great longing came on him to go back
to Ireland. Niamh did not want him to go but at last she agreed and
gave him the white horse. Niamh warned him “set foot, even once, on
the soil of Ireland and you will never return to Tír na nÓg.”
When Oisín reached Ireland he found that everything had changed. There
was no trace of his father or the fianna. As he passed through Gleann
na Smol, the valley of the thrushes, he saw a group of men trying to
move a large stone. “I will help you” he said. The mighty Oisín
stooped down in his saddle, and with one hand, lifted the stone. But
as he did so, the saddle strap broke and he tumble to the ground.
Immediately the fairy horse galloped away and a great change came over
Oisín. In the blink of an eye the great hero of the Fianna became a
withered old man.