THIS REVIEW IS MEANT TO BE READ AFTER YOU'VE SEEN THE EPISODE. I WON'T BE GIVING A PLOT SYNOPSIS FOR THE EPISODE AND I WILL BE REFERENCING SPECIFIC EVENTS, SO IF YOU HAVEN'T SEEN THE EPISODE THEN THIS WON'T MAKE MUCH SENSE.
Irrational Treasure is probably the silliest episode of Gravity Falls. It's one of the only ones that treats its supernatural element(if you can call peanut brittle supernatural) with no sense of danger and only uses it for comedy. This is both the episode's biggest strength and its greatest weakness, making for some of the funniest jokes in the series.
Like I said, this episode is funny. Like, incredibly funny. It's easily one of the funniest episodes in the entire series and a lot of that is due to the pure wackiness of it. Gravity Falls is always weird, but it's clearly based in the real world. There are very few obvious Loony Toons style gags that break the laws of physics for comedy, the police get called when something terrible happens, and there's a general sense that the supernatural is still unusual and not an accepted part of everyday life for most people. Irrational Treasure forgets that in favor of sending the Mystery Twins on an adventure based purely on what's funny. The jokes come a mile a minute, largely due to the fact that every possible funny element is present. Stan gets a hilarious b-plot, Dipper and Mabel have wonderful banter, and the side characters and environments all get their own fabulous jokes. The show's comedy is firing on all cylinders throughout the first two thirds of its run, then it introduces Quentin Trembley and the entire thing goes into overdrive. He is easily the funniest side character the show has ever produced and he steals every scene he's in. His dialogue is perfect and it's obvious that Alex Hirsch is having a blast playing the character. Every line just oozes clever writing and energy. It's impossible to not laugh at everything the man does and says, making the last third's comedy near perfect. It helps a ton that Dipper and Mabel work perfectly off him, Mabel letting his silliness go wild and Dipper providing a practical bedrock/straight man like he normally does with his twin. All these elements combine in a perfect blend that makes Irrational Treasure an incredibly funny masterwork of comedic writing and performance for Gravity Falls.
As the comedy is so great it covers up the other parts of the episode, good and bad. Mabel gets her own arc, which is nice and makes sense for the character, but it isn't given nearly as much time or emphasis as her arc from The Hand That Rocks the Mabel was. It does a good job of letting Mabel learn a lesson and do her thing, but it doesn't feel that special, mostly like it's just doing slightly more than usual with the character. It's a welcome change from her usual place as second fiddle to Dipper's emotional arcs, but its underwhelming compared to the other development she's gone through in the show before.
Really the only thing holding Irrational Treasure back is that it doesn't have a large enough emotional center to make it an incredibly important feeling episode. The show's best episodes have great mystery, character development, and humor, and Irrational Treasure manages to do a slightly above average job with two and an incredibly fantastic job with the third. It's really, really, really funny, but like many episodes of season one it lacks an exceptional emotional base to make the entire thing feel special. That doesn't hold the episode back from being a hilarious piece of comedy that's definitely worth watching, but it does mean that it can't be the best that the show truly has to offer.
-I don't hate Pacifica nearly as much in this episode as I did in Double Dipper, but that's mostly because she has very little screen time and Dipper's reaction to his revenge on her is great. The "revenge feels awesome" message is surprising for a Disney show and like the frequent supernatural and adult elements it helps keep Gravity Falls feeling fresh compared to its competition.
-I really love that guy from the government report. His reaction to learning he won't be shot is just gold.
-(SPOILERS FOR LATER IN THE SHOW'S RUN) I'm still waiting for Quentin and the president's key to show up later on in the show in some important capacity. Hopefully the finale gives us something.