Hareth = "forest", pronunciation Chereth {kheh'-reth}. Lexicon Results for Chereth (Strong's 02802) 1Sa 22:5 Forest of Hareth. An example of J as H pronouncement as in Josa or Hosa.

I have read that it is a name in its own right, from Celtic derivation meaning 'Strong warrior'.

A Celtic name meaning 'Strong warrior'?

This name is neither Celtic nor means anything of the sort; it is an atrocious blend of two names that are wonderful on their own in their undamaged form.

Jarlath is the Irish name you are probably thinking of; this is from babynamesofireland.com:

"St. Jarlath (born c. 550 AD) was noted for his piety and his ability as a teacher. In old age he decided to found a monastery where he could end his days. He asked one of his pupils, St. Breandan the Navigator, to drive his chariot east and when the chariot broke a wheel at Tuam in County Galway he took it as a sign that that was where he should end his journey, founding a church that became a great center of learning and art. The name is still popular in this part of Ireland."

I named my son this because it is unusual, yet easy to spell - and very masculine. He is now eleven, and still loves the name - particularly since seeing 'Labyrinth' where David Bowie plays his namesake

Jareth was the name of the Goblin King in Jim Henson's 1986 movie "The Labyrinth."

It's also considered Italian.

Instead of only being a blend of the names Jared and Gareth (Seth is also a possibility, by the way), Jareth could also simply be a more creative spelling of Jared (and thus a variant spelling, one could say).

This is also the name of one of the characters in a book by Jane Yolen, White Jenna (maybe Sister Light, Sister Dark too but I can't remember for sure or not).