Happy St. Paddy's!
The only people whom the Romans had failed to ever conquer were the Celts of Hibernia (Ireland), and the Celtic Picts (“painted people”) of northern Scotland. They tried invading for over 50 years before giving up on Ireland, and they literally had to build Hadrian’s Wall to keep the crazy-ass, tattooed Picts from coming down from the highlands and regularly delivering brutal Roman smack downs.
Here are some quotes about them from famous historians of the time:
"The Romans… were terrified by the fine order of the Celtic host, and the dreadful din, for there were innumerable horn -blowers and trumpeters, and… the whole army were shouting their war-cries… 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 torcs and armlets." -Polybius, 2nd century BC
"The whole race, which is now called Gallic or Galatic, is madly fond of war, high-spirited and quick to battle, but otherwise straightforward and not of evil character. For at any time or place and on whatever pretext you stir them up, you will have them ready to face danger, even if they have nothing on their side but their own strength and courage." -Strabo, Roman historian.
"We have no word for the man who is excessively fearless; perhaps one may call such a man mad or bereft of feeling, who fears nothing, neither earthquakes nor waves, as they say of the Celts" -Aristotle
"Their armor includes man-sized shields decorated in individual fashion. Some of them have projecting bronze animals of fine workmanship… On their heads they wear bronze helmets which possess large projecting figures lending the appearance of enormous stature to the wearer. In some cases horns form one part with the helmet, while in other cases it is relief figures of the foreparts of birds or quadrupeds. Their trumpets again are of a peculiar barbaric kind…and produce a harsh sound which suits the tumult of war. Some have iron breast-plates of chain- mail, while others fight naked." -Diodorus Siculus. 1st century BC