On the mundane side, they're highly proficient counselors, and possibly psychiatrists. Minor problems don't even require them to burn a spell slot. (In the same way that a more traditional healer is probably proficient in mundane Medicine in addition to being able to cast Cure Wounds - Irendif is associated with Insight instead, and maybe Persuasion.) Depending on how grim and gritty I want to be about premodern understanding of psychology, there's probably a decent mixture of folk medicine and stuff in there that dubiously works, but I don't generally go for too much anthropological accuracy in my stories (too much of an idealist). I figure they're pretty good at this aspect of their job.
But the aid of magic opens up a world of possibilities that aren't available to real-world therapists.
- With consent and careful use, detect thoughts could make a cleric of Irendif preternaturally good at identifying a patient's actual issues and figuring out treatment for them, complementing a mundane skill for insight and understanding.
- Greater restoration is standard treatment for serious madness in adventures like Out of the Abyss, and they're probably even able to use lesser restoration to heal minor disturbances or delusions.
- Modify memories could be used to take the edge off of the traumatic memories of war veterans, for which reason an army classically wants at least one around - the temple probably objects to removing memories entirely, for reasons of doctrine, but they don't mind altering them to have a softer impact.
- Calm emotions could probably stop an acute panic attack, though magic isn't available in great enough quantities to make this a viable treatment for prolonged anxiety issues (the spell only lasts like 10 minutes).
- Traumatic brain injury might even be curable by classical healing, but if not Irendif definitely has it covered.
In general, I figure that if magic can heal damage to the rest of the body, it can probably help re-balance one's brain chemistry and heal brain injuries as well - and Irendif's followers would specialize in it.
(The fact that Irendif's doctrine is unambiguously Good-aligned, affirming the value of individuals and their power of choice, is something the rest of the world should be grateful for. These guys would be flipping terrifying if they were evil.)
Critically, Irendif is the Namer, and her priests share this title and ability. This means that they are capable of recognizing a person's "true name" (how literal this is is a matter of some ambiguity) and thus gaining an understanding of their identity. In minor cases, this merely makes them capable of recognizing aspects of the patient's character to help guide them. In extreme cases, however, this means they are capable of breaking memory loss on the spot: I once wrote a short piece in which the high priest of Iridis (her name at the time) is able to discern the name of an amnesiac Singit, the god of magic, and restore him to godhood. She also regularly reminds the hunter-god Orhansin of his name to stabilize the wild shifts of identity and temperament that come with his nature as a fey lord: her influence keeps him centered and focused, something that most of the fey are not generally capable of.
They also have an important narrative role in classic Inraget stories. Telmadion and Verhusaf often sought their mother's counsel before making decisions, and she was an important linchpin in keeping the gods from succumbing to despair in the darkest moments of their war with the Wild Lords. Priests of Irendif make appearances in certain stories from the Western epics, serving a similar role for mortal heroes: to encourage them, clear their minds, and send them on their way more ready to face their adventures.
In a couple of stories centering on Namers, they are even known for redeeming villainous characters. The climactic moments of the LEGO movie, Return of the Jedi, and the original Percy Jackson series would all be classical Namer moves - recognizing someone's true nature and respecting them enough to steer them toward the light. This came into play in one of my recent campaigns, where the characters turned over a delusional vandal who'd been working for the BBEG to the temple of Irendif for treatment, and he later reappeared with a stronger grip on reality as an important informant for them in the last stages of the campaign. (That party had a thing for redeeming villains in general. It didn't always work.)
And when that fails - well, the advice of a Namer can also be a big help in bringing an unredeemable villain to ruin. But many of them will refuse such aid on general principles.
One part psychiatrist-plus-brain-surgeon with magical aid, one part adviser and guide, one part Knower of Names, and one part wise old man archetype.
Aaaaaand there were other things I had planned to do tonight, but I wrote a lot more than I planned to, so yeah.