Gaesatae in the context of "Insubres"

Over the course of nearly four centuries, the Roman Republic fought a series of wars against various Celtic tribes, whom they collectively described as Galli, or Gauls. Among the principal Gallic peoples described as antagonists by Greek and Roman writers were the Senones, Insubres, Boii, and Gaesatae.

The Romans first came into conflict with Gauls who entered Italy from the north. Some of these settled in the lands immediately south of the Alps, which became known as Cisalpine Gaul: "Gaul this side of the Alps". Gaulish armies, some perhaps fighting as mercenaries in the service of the cities of Magna Graecia, plundered territory in Etruria and Latium during the fourth century, famously sacking Rome circa 390 BC.

Viridomarus (or Britomartus as translations vary; died 222 BC) was a Gallic military leader of the Gaesatae. In 222 BC he was hired by the Insubres who were fighting the Romans. Viridomarus led a mercenary army against a Roman army at the Battle of Clastidium. The Romans won the battle, and in the process, Marcus Claudius Marcellus, the Roman leader, earned the spolia opima by killing Viridomarus in single combat.

The Battle of Clastidium was fought in 222 BC between a Roman army led by the consul Marcus Claudius Marcellus and a Celtic force of the transalpine Gaesatae, called by the cisalpine Insubres. The Gaesatae were led by Viridomarus (Florus) or Britomartus (Plutarch), as the name varies in translation. The Romans won the battle, and in the process, Marcellus earned the spolia opima, one of the highest honors in ancient Rome, by killing the king in single combat. It was also notable as one of the few battles won by the Roman cavalry without any aid by the legions.

The kings of Leinster (Irish: Ríthe na Laighean) ruled from the establishment of Leinster during the Irish Iron Age until the 17th century Early Modern Ireland. According to Gaelic traditional history found in works such as the Book of Invasions, Leinster was created during the division of Ireland among the Irish Gaels, descendants of Milesius: Leinster was one of the territories held by the offspring of Heremon. In the 7th century BC, the branch of the Heremonians who would establish Leinster, starting with Úgaine Mór, were also High Kings of Ireland and Kings of Tara. Their ascent to hegemony in Ireland was associated with the decline in influence of their Ulster-based Heremonian kinsmen from the Érainn.

Apart from Úgaine Mór, other prominent Kings of Leinster from this period who were also High Kings of Ireland were Labraid Loingsech and Cathair Mór. A mythology developed that Labraid Loingsech had horses ears: he spent some time exiled in Transalpine Gaul (dated roughly to the period of the Roman–Gallic wars) where his grandmother was from and returned to Ireland with Gaullish mercenaries. He established a base in the area, which was renamed from "Gailian" to Leinster, in reference to the pointed-spears held by the Gaullish Gaesatae mercenaries who provided the backbone of Labraid Loingsech's powerbase and brought him to the High Kingship. Cathair Mór, who was also a High King of Ireland, is perhaps the most important figure genealogically in Leinster as all of the subsequent kinship groups which ruled Gaelic Leinster claimed descent and legitimacy to rule from one of his ten sons who had offspring.