You seemed to 'know well' that Firbolg were 'mythical', when in reality they were Belgae in Hibernia. The Gauls invented many, many things the Romans (and Greeks, and numerous other societies) adopted. Just because they didn't write doesn't mean they weren't military innovaters. They were, in fact, quite intelligent.
Rome was not sacked by a horde of undisciplined imbeciles. It was sacked by the Gallic equivalent of Hannibal, Alexander, or Caesar, a military genius who obiliterated the northern forces of Rome (maybe a tad exaggerative of his prowess, but he was clearly a great leader and general). Brennos was not a man commanding hordes of unwashed barbarians, he commanded a trained, intelligent force, with professional warriors.
Roman and Greek authors write much of Gallic tactics and war-like behavior, but they also tend to embellish things. Livy said the great hardships Gauls endured, when serving Carthaginian armies, then immediately called them lazy. The Gauls sacked and humiliated Rome, Roman opinion is more than a little biased against them. It's not that Gauls had elaborate legions or what not, but they were not undisciplined hordes. They were combinations of young, inexperienced warriors, who probably acted in hordes, mercenary "tribes" who didn't feel the need to 'advance' with times (Gaesatae), and true professionals. The Romans changed their manner of war because of Gauls, that means that Gauls had to have truly impressed the Romans with their war against them.
It has nothing to do with nationalistic pride, I'm not a Gaul. I'm a Gael, and they fought significantly different (combinations of Gallic and British tactics). I have respect for Roman militarism, but the Romans adopted things constantly, that was what made them good. They would take the better aspects of their enemy's militaries, and make them their own (such as Gallic helmets, Spanish swords, etc.).
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