The warriors were not a social class of people. All the ancient Celtic people were warriors if they needed to be. Although most women stayed home, looking after the crops and the children, when their men went off to war, women could choose to train and fight as warriors if they wished.
There were warrior schools in the world of the ancient Celts. A couple of those schools were run by women. But they were the exceptions. Most of the warriors were well trained men.
You might hear tales of warriors running naked into battle with only a shield and a sword for protection. There was one tribe that painted their body blue and ran naked into battle. But most warriors wore some protection. The nobles wore chain mail, and carried a shield and a sword. Peasant's shields might be made of wood instead of iron, but they were covered in leather. Their weapons were made of iron.
Celtic warriors used many tricks to scare their opponents. The Celts were naturally tall, with blonde or red hair. They rode horses into battle, which not only gave them a fighting advantage, but made them look even taller. Many warriors wore bronze or wood helmets shaped with two horns, that made them look taller yet.
Men put lime in their hair before a battle. Lime burns and acts as a bleach. It lightened their hair so it was even more blonde. The lime also stiffened as it dried. This gave the men a protective glowing crown. Just before entering a battle, musical horns were blown. Men beat their swords against their shields and screamed battle cries and insults at their opponents.
The Celts like to chop off the head of the leader of their enemies. That head was cured and rubbed with oil, and nailed over the doorway, somewhat like modern hunters do with deer heads.
If you were facing an army of Celtic warriors, you would see a seemingly endless field of tall, screaming scary looking god-like creatures with lime spiked hair glowing brightly in the sun, and severed heads hanging from their belts and wagons. It would have been a most terrifying sight. The Celts were not only fierce fighters. They looked and sounded like fierce fighters. They often won, sometimes without even entering into battle. Between trading and raiding, the Celts soon became rich.
Before long, the Celts had taken over nearly all of Ireland, Scotland, Wales, and the Isle of Man. The other tribes in these areas either moved away, were killed in battle, or joined the Celts. When the ancient Celts ran out of other tribes to fight, they began to fight each other. Close relationships between clans did not stop them from raiding each other - stealing cattle and other wealth. This led to many a battle.
Various clans rarely went into full battle with each other. What they would do is show up in full battle dress at a pre-arranged location. Each side would line up at some distance from the other, and scream insults at each other. Each side made as much racket as they could. They beat their shields and screamed and shouted and blew horns. Finally, when they had just about exhausted themselves from all their noise, each side would send a predetermined number of men - one or two or a handful - forth to fight for them. These fights were usually to the death. The outcome of the fight determined who was the winner. Goods or lands were distributed accordingly. Then, the two sides went home.
The ancient Celts had always fought as a clan, not as a people. Clans might team up against another clan or two, but they had never banded together totally to defeat one common foe.
When the Roman army arrived, the Celts did not band together to fight them. The well organized Roman army found them easy prey. The Romans did not wipe out the Celts. But they did rule the Celts for many years.