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.
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
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).