From the Dr. John Christopher School of Natural Healing - Herbal Guide, there are four herbs which he specifically calls out in index as beneficial for poison ivy and poison oak - lobelia, sassafras, slippery elm and virginia snake root. The latter for which I have no knowledge of or experience with I will not comment on.
I posted about a month or so ago about having found a small crop of sassafras bushes. One of them had a young healthy shoot of poison ivy that was literally growing wrapped around one of the bushes, but that poison sprout has since been taken care of. Dr. John advises sassafras can help with poison ivy by being taken both internally as in a tea as well as used externally, like a topical solution dabbed onto poison blisters at regular intervals while the condition persists. In general, sassafras is a blood-purifying herb with various anti-poison properties. Ideally one wants to use sassafras for which they know the quality of, not necessarily an easy task. Wild/natural sassafras is not very easy to come by commercialy since the fda has endeavored to regulate it since the early to mid 1900s. From their point of view, the Safrole ingredient of sassafras is dangerous to humans since their scientists succeeded in killing white rats in a lab after exposing them to large doses...... whatever "large doses" may have meant in those cases.
Dr. John devotes an entire chapter to the many and varied uses of Lobelia. One really needs to read this chapter to appreciate the scope of uses. Specfically with regards to skin problems (such as but not limited to poinson ivy/oak), he suggests a simple poultice comprised of 1 part lobelia and 2 parts slippery elm.
As for slippery elm, it's medicinal uses are also many and varied, including benefits for skin eruptions. An outbreak of poison ivy and oak definitely falls under this category. He provides a long list of ingredients he uses as his "asthma ready" formula that he also advises can be used for various ailments, including poison ivy, in which case this formula is applied to the affected area as a fomentation, which is kind of like a poultice, except it's usually applied via a soaked fabric, like cotton or gauze, and it's effects can generally be improved with the use of heat or through cycling the application of heat, then cold, then heat, then cold, etc, to the affected area.
Plantain was also a favorite herb of Dr. John's. While the guide does not specifically call out it's use for the treatment of poison ivy/oak, I do not see how it's use could hurt since it does have anti-poison properties that are beneficial for treatment of conditions such as poisonous stings & bites, as well as relieving conditions of swelling, blood poisoning and severe infection that otherwise could lead to amputation of severely infected limbs and body parts. In these instances plantain is applied as a basic poultice, but there it can also be brewed into a tea and taken internally. (IE> infusion) Plantain is so commonly available, I've now adjusted grass mowing habits to avoid cutting down plantain plants wherever I see them in the yard. At least during the spring and summer months, I'll always have some fresh growing plantain available as may be needed and will probably attempt to preserve some for the winter months.