Short answers, pulled from the story.
The Battle of the Allia took place in 390 BC according to Livy's Varronian chronology, though Polybius derives 387 BC using a Greek dating system and Plutarch suggests a date shortly after 393 BC based on lunar observations.
Brennus was the chieftain who led the Gallic tribe known as the Senones into northern Italy and commanded their forces at the Battle of the Allia near the confluence of the Tiber River and the Allia brook.
The battle occurred at the eastern juncture of the Tiber River and the Allia brook located 11 Roman miles north of Rome where the Romans failed to set up camp or build a defensive rampart before engaging the enemy.
Three sons of Marcus Fabius Ambustus violated ancient neutrality rules by killing a Senone chieftain during negotiations which caused the Gauls to withdraw and later attack Rome directly.
Modern historians estimate that both sides had around 5,000 men rather than the tens of thousands recorded by ancient sources like Livy and Diodorus Siculus.