As warriors the Celts were very fierce.  The women held some renown also as they fought side by side with the men.  It is in Ireland where great tales, like those of Cuchulainn tell us of the Irish fighting code.  Incredibly it was not uncommon to have whole armies wait for battle on the field of champions deciding the interior conflict and appropriate course of action before these barbarians went at it.

The Celts had a good knowledge of iron and made their weapons and tools of the strong metal. 

They even looked the part of the fierce barbarian with limed hair and often painted mystical designs in blue woad (blue dye from a plant in the mustard family) on their bodies.  Another thing that made them feared (heck it's still scary today) was the practice of taking trophies from the field of battle in the form of their enemys heads!

They were renowned as great horse warriors too striking with javelins and their long swords riding on horseback or from their light and swift wicker chariots.  They could alter their battle tactics to fight many different ways making them hard to dominate.  It took the supreme military might of Rome to finally break the strong Celts.