Thanks! Of course I've trained Irith (though most times activated the portal long before I really raised anything but mood (which promptly goes back down so it's meaningless to raise it before), because knocking down debt to something manageable took priority, and I do portal and torture chamber long before that as well, because it doesn't demand nearly as much grind), paid off the guild, I doubt the issue lies there - and walkthrough doesn't mention either as prerequisites for portal defence, anyway, but might as well try to do all that training and grind a bunch before the portal, if only to have saves to restart from.
I think problem rests somewhere around the trigger for “Defend Portal” option: either you shouldn't activate the portal in some specific conditions that I keep reenacting, shouldn't read about portal defence until you do everything else as otherwise it gets marked as read in the research topics but doesn't get you Defend Portal option, or you shouldn't experiment with crafting until you get all recipes from the books, or you should hold off activating the portal until you get and do everything that you can do without it done.
Least that's what I'm going to try on my fifth try (paying off while still saving something for gifts and training yet AGAIN and without advanced crafting this time's gonna be a pain in the ass, but what can you do, using rollback is as far as I'm willing to go, QoL-wise).
Anyway, the most annoying bit about it is not that it's unintuitive, but that all your grind is wasted over it, and I've yet to figure what I'm doing wrong.