I never got poison ivy for the first 25 years of my life. I just don't think I ever came into contact with it.
Then I bought a house with large backyard with lots of small trees that needed removal. After the small trees were gone, I tackled the briar and other vines with thorns. That's where things went awfully wrong!!! It was fall and there were no leaves on the trees. There were these huge fuzzy vines up several trees so I pulled them down with my bare hands...a few days later, I had poison ivy everywhere. I was miserable for ten days. I tried everything and nothing worked for more than a couple hours.
A few years later, I got it again. Still not sure when or where. I did some serious internet searching and I found the miracle treatment...Hot Water.
While taking a shower, I get the water as hot as I can possibly stand. I mean 1/10 of a degree before scalding. I then spray the hot water on all the poison ivy spots. They will itch so bad you probably won't be able to stand it but after a few seconds the itching will go away entirely. I'll then move to the next spot and the next. I'll then start over and keep doing this until the itching is totally gone. It'll take about 15 minutes or more if you have many spots.
I find that with this method, it's best to make sure all the blisters have been popped. They also seem to heal faster if they are popped.
For me, the itching doesn't start coming back for about 6-12 hours. I like to hit them with hot water again just before bed so I don't get woken up scratching in my sleep.
The theory is, histamines rush to the site of the hot water which is why it itches so bad. The histamines are then flushed from body. It takes a long time for the histamines to build up again which is why you have many hours of relief.