More than you ever wanted to know about the stuff:
Basically, the author reviewed 69 papers and reports and gave a summary.
Start with page 21 if you don't want to slog through the whole thing.
I've had trouble with poison ivy since I got a monster rash with huge blisters as a little girl. Naturally it grows like crazy here. The birds eat the seeds and then hang around my trees and shrubs. It's a never ending battle to eradicate it while still a small seedling.
If you don't wash the urushiol off quickly enough, there's no way to get rid of the resulting rash/blisters or shorten the process. Hang tough, in all my life I've never had it last longer than 2 weeks.
The best way I've found to get relief from the itching is to use cold or heat. Ice the rash until the skin is numb. Or either run hot water over it or hold a hair dryer close to the rash long enough to sting (but not long enough to burn.)
That and extra strength anti-itch cream with 2% diphenhydramine HCI works, especially in the mornings. The generic version works just as well as the expensive stuff. Cortisone has never worked for me but maybe it works for milder cases, don't know.
As the paper states, antihistamines don't do anything for the itching, but they were the only thing that made it possible for me to sleep during the worst of it.