I recently made a fresh batch of old fashioned lye soap for my customers who request it to use on poison ivy outbreaks. I have personally never had poison ivy, but I understand that applying a paste made with lye soap will dry it up quickly.
Here are the instructions for using it on poison ivy or poison oak:

For Prevention: Bathe within an hour after contact.
For Treatment: Mash a small amount of Lye Soap with warm water, and apply to affected areas with a cotton swab. Dispose of swab after use.
Do Not Put Contaminated Swab Back On Soap. The oil from the poison ivy or poison oak will contaminate the soap and spread the dreaded itching.
Do Not Wash! Leave paste to dry on skin. Reapply as needed after gently rinsing with cool water and cider vinegar.**

**These statements have not been evaluated by the FDA. They may provide temporary relief, but are not intended to diagnose or treat any medical condition or to replace the advice of a health professional. As with any product, if you experience a negative reaction of any kind, discontinue use immediately.

One of my customers is a landscaper and she also recommends a product called Ivy Dry. It’s available at most major drug stores and grocery stores.