It isn't Christian nor Celtic. The oldest examples of the "Celtic" cross are those engraved or painted on flat pebbles, dating from 10,000 BC and found in a cave in the French Pyrenees. These "ancestor stones" were believed to contain the spirits of the dead.
In Scotland, The stones at Callanish are laid in the shape of a Celtic cross. Callanish also predates Christ. It is possible the Christians took the cross symbol from the Celts or Megalithic peoples but certainly not that the Celts took the symbol from the Christians.
Information on megalithic sites is available at
http://www.geocities.com/SoHo/2621/stones1.htm
Callanish info at
http://www.geocities.com/SoHo/2621/callanis.htm
Further information on the Celtic cross and its Christian symbolism
is at
http://www.geocities.com/Athens/3374/