I just found a MASSIVE poison ivy patch

[usonian]

3. Been there, done that.

Firefighters carry Dawn detergent with them to dissolve the oil on the spot.

Later, you can use deodorized mineral spirits (I think they are ALL deodorized these days, because you can get high as a kite on mineral spirits). That dissolves the oil. Wash it off gently after a couple of minutes.

Commercially, Tecnu is just that.
https://en.wikipedia.org/wiki/Tecnu
It's undoubtedly safer than the paint thinner mentioned above.

Vinegar (the hardware store kind, not the kitchen kind) can be used to kill the poison ivy/oak plants if you don't want to use any weed killer.

If you can afford it, hire someone. Hard to believe, but some people are immune to the oil.

and DO NOT burn the plants. That just vaporizes the oil.