The problem was that Giles was just plain wrong. That, at least, was Leo's opinion in the matter. (To forestall any further speculation or argument, Leo's opinion is the correct one in this instance.)
Which is not to say that Giles is not or has not been right on many things in his lifetime. It's a fact universally acknowledged that Giles has an enormous brain with absolutely top notch mental faculties installed inside it.
But on this particular occasion Giles was headed down Wrong Turn Alley without a map. To put it bluntly, he didn't know what the hell he was talking about.
Granted, Leo could understand the other man's argument. They had limited contact together due to being in completely different Guardian Duty circles but when occasion called for them to join together in knowledge; they did so and were a force to be reckoned with.
Still, there was a very valid and good reason for them not meeting much. That being the fact that the Slayer was a relatively new phenomenon and the duty of watching over said Slayer had been given over to the Council. (An extremely poor decision on the part of the Elders, mind you.) Thus, Leo had nary a reason to watch over a Slayer and Giles had nary a reason to assist the Charmed Ones.
But demons, villains, vampires, warlocks, prophecies, and other assorted plots that assist good television scripts often conspire to make things difficult for the average good guy. So Giles had made at least two separate journeys to San Francisco to get help and/or advice from Leo and Leo will certainly never forget his time in Sunnydale during the Bacon-and-Egg Spell Disaster of '02.
While Giles insisted the Nexus under the Manor was really a Hellmouth, Leo could forgive the British Watcher such small quirks because the moniker would indeed be well-earned should the Manor come under the control of evil. As it was, it sent out supernatural vibes like a cat in heat.
Leo could forgive many of the things Giles said regarding different theories, titles, and combat. But Leo could not, would not, absolutely refused to accept the lack of leprechauns in the world. It simply would not do.
Had he not conversed with them on many an occasion? Had he not traveled to their lands, been doused with their luck? Did he not have leprechaun allies and friends?
In case there was any doubt, yes, he had and has.
And Leo was a patient man. He was married to Piper Halliwell for goodness sake. If that wasn't recommendation enough for the patience of a saint, nothing is or ever could be. Giles knew it too. He'd met Piper. He'd met Piper during the Chupacabra incident. Incidentally, they had become good friends after, but during Piper had tried to kill Giles no less than six times.
Which just goes to show that Giles also was a forgiving and lenient man. He had proved such on many an occasion when the frequent American teenage hormones around him would have proven too much stress for any normal British Watcher.
And just because the Elders had come before the Council didn't give Leo any reason to think his knowledge was any grander than what Giles possessed.
Leo had been forced to concede in the matter of the Trgelsi demon. And that one time Giles had been orbed to Paris.
That was the last thought through his head before the real arguing started.
Based on aforementioned evidence of the reasonableness of these two men, it was rather a startling effect when the shouting began.
Giles had taken his glasses off earlier in the discussion and was now rubbing them furiously on his shirt. Leo was starting to orb around the edges through sheer frustration.
They were almost at the point of:
when Piper came down the staircase and froze Giles leaving Leo yelling at a statue.
Which made him feel extremely silly.
"Why don't you just take him there?" Piper asked rationally and didn't it just gall that she was being the rational one in the situation? "Don't forget it's your turn to pick up the kids."
And she waved her hand back over her shoulder as she left.
But Leo was (as previously stated several times) a reasonable man, so when Piper unfroze Giles, he simply turned and walked up the stairs to the attic, grabbed the stick given to Paige and shouted.
"Go n-éirí an bóthar leat."
Giles, having followed, rather aghast at such a cowardly exit, stopped talking, his mouth gapingly rather unattractively as a rainbow appeared in the attic.
"Coming?" Leo asked, extending a hand.
Giles wasn't really capable of words at the moment, but he gamely stepped forward and away they went.
It only took about five seconds for Giles to start rubbing his glasses again, but both men had calmed down to less dangerous levels of knowledge smug-ness.
"Now that leprechauns are real…" Giles began as if Leo had only just invented them and, therefore, all his arguments before had been valid and not foolish in the slightest "…I'd like to know some more."
Leo, (really a very generous man), graciously let Giles read the Book of Shadows and its entry on leprechauns.
Giles left the Manor mumbling under his breath about luck statistics and the impossible shades of green he'd seen and was already planning a treatise to share with Wesley when next they met.
Leo picked up the kids.