Blue tansy is typically recommended for things like insect bites and other irritations of the skin. Tamanu and/or virgin coconut oil as a carrier may both also help. Simple lavender may even do the trick, but Blue Tansy is the first that comes to mind -- from a lot of research on essential oils and skin care.
Blue tansy is recommended for allergic reactions, and may be the best choice because poison ivy is somewhat of an allergen. Other oils to consider (the percentage is if you only used them alone in carrier - 1% is 35 drops essential oil per ounce of base):
German Chamomile (2%)
Cape Chamomile (3-5%)
Sea Buchthorn (3-5%)
Calendula (1%)
I've been able to apply blue tansy "neat" on small areas without a problem -- for larger areas, a carrier should be used, and again,
both Tamanu and virgin coconut are very anti-inflammatory and soothing. 
