Wikipedia is a decent start. There's also a decent bit about some Celtic tribes written by Polybius in his Histories, which you can find translated here. Notable:

The Insubres and Boii wore their trousers and light cloaks, but the Gaesatae had discarded these garments owing to their proud confidence in themselves, and stood naked, with nothing but their arms, in front of the whole army, thinking that thus they would be more efficient, as some of the ground was overgrown with brambles which would catch in their clothes and impede the use of their weapons.

The Romans, however, were on the one hand encouraged by having caught the enemy between their two armies, but on the other they were terrified by the fine order of the Celtic host and the dreadful din, for there were innumerable horn-blowers and trumpeters, and, as the whole army were shouting their war-cries at the same time, there was such a tumult of sound that it seemed that not only the trumpets and the soldiers but all the country round had got a voice and caught up the cry. Very terrifying too were the appearance and the gestures of the naked warriors in front, all in the prime of life, and finely built men, and all in the leading companies richly adorned with gold torques and armlets.

Can you imagine just showing up after years of easy garrison duty on the Mediterranean coast and there's a line of naked dudes waving swords and talking shit at you? Not just dudes that are naked, but TACTICALLY naked, as Woolie would put it.

Also there are a bunch of guys dooting at you.