Thanks for the chapter, FlamingSquid.
Tarte's are way better.
That said, why should anyone let his guard down around a snake? Regarding Naoise, Lugh's dismissal is careless--the guy (Naoise) is following one part of the human nature playbook to the letter: ego over logic, emotion over rationality. Can't do a thing against demons, but still wants to throw his nose into the air and fight alongside someone who can. Once denied, his feelings will be hurt (injured pride), and so he'll either want to retaliate or otherwise do something that'd sabotage Lugh's efforts--with or without intent.
Lugh could have just played in Naoise's head and told him to screw off (without telling him to screw off) and fight demonic footsoldiers (i.e., the monsters the demons are sure to field) when the time comes.