Ok I know I'm like 4 months late and a lot of people have probably said it before me, but I need to talk about it or else I'll explode. SO here's why the Woe's Hollow episode is the best shows Irv's development (and his personality in general) in 3 points:
1. He no longer trusts Lumon with their safety
When the MDR team are dropped into a forest in the middle of nowhere, they stumble across a dead seal. Irv immediately suggests they eat it, explaining that they don't know how much longer they'll be out there. We've gone from a guy who doesn't hesitate for a second to let himself fall into Milchick's arms as a test of trust, to a guy prepared to eat the corpse of a wild animal in order to survive.
2. He's lost interest in Kier's history and the creation of the company.
This ties in a bit with the first point, but yes, Irv has completely lost faith in Kier. When Milchick reads them the story of Kier and his supposed twin brother, Irv is stunned and doesn't hesitate to ask questions. Before Irv met Burt, he was fascinated by Kier's mythology and knew every principle by heart. But, as @beetleandfox said so well in her post on the parallel between Irv and Helly, his faith in Kier isn't innate; his thirst for knowledge and his love of art are. What's more, Irv is a very intelligent person. Now that he has realised that Lumon takes advantage of their naivety and that the innies have virtually no control over their own lives, he is no longer blinded by his faith and is becoming more attentive to his environment and, above all, more objective.
This is where we get to the most important part of Irv's character. After Burt's 'retirement', Irv becomes the stronger version of himself. He's more analytical and takes matters into his own hands. What's more, he knows how to keep his cool, even when it hits a nerve. When Helena makes the inappropriate remark that he'll probably never see Burt again, he isolates himself from the others. He's hurt, yes, but that doesn't stop him from realising that Helly would never attack him in such a sadistic way and that something was wrong with her. Her lack of empathy on his return only confirmed his doubts about her. When he realises that they've been with Helena all this time, he's ready to drown her rather than let her steal what little life Helly has, even if it means losing his own.
And yet... he's terrified. Terrified that he might be wrong, or worse, that Milchick won't intervene and let him become his friend's murderer. So when Helly comes back, Irv takes her in his arms, crying. He knows that, even if she had nothing to do with the situation, it will be harder to trust her now, but his affection for her is even stronger than his intelligence.
In conclusion, as well as being an intelligent and dedicated character, he is also highly sensitive, both to his environment and to those around him. He has a deep desire for knowledge (which he shares with his outie), but the lives and dignity of his next to kin are even more important to him. And this episode is a real credit to him.