The allergen in poison ivy is a substance called urushiol. What urushiol does, to people who are allergic to it, is bind with the skin cells and produce a rash. In order to treat a reaction to the poison ivy what you essentially need to do is “neutralize” the urushiol.
There are several herbs, when applied topically, that can do this. Jewelweed is one of the best. You can also combine this with any herb that contains a significant amount of saponins such as Soapwort, Horse Chestnut, Licorice, or Rose Leaves. Please remember that these are for topical use only as saponins shouldn’t be taken internally while pregnant or nursing, but are perfectly safe when used externally. Some things to help control the itching are aloe vera or plantain. Also, cool baths with powdered colloidal oatmeal can be extremely soothing.