Imannae generally act as a screen before the main force, loosening the enemy formations before the older warriors charge.

Expert at Hiding in Woods
Can Hide in Long Grass

The Imannae (Im-an-ay; "Ambushers") are youths used throughout many less developed Celtic territories to ambush and disorient enemies. In an army, they generally act as a screen before the main force, loosening the enemy formations before the older warriors charge. They are actually more akin to a militia arm, and are employed by tribes in dire need of defense; they are best employed as hit-and-run soldiers. If left into a melee, they will break and flee.

Historically, the Celts had to rely heavily on skirmishers when their armies were weakened. However, they also used them to weaken campaigning and marching foes, and as a light warband for raids, as well as screen their armies. They were very prolific ambushers; most Celtic territory was wooded with many places to launch such attacks from, and these types of soldiers would be of most use in similar terrains. As young men still trying to raise their status, it is unlikely they were very sturdy in a prolonged battle. It was not unusual for such 'warriors' to take up jobs as mercenaries, to ensure better pay than they could normally get for their station, and some probably even went into outright banditry.