Here is what Perdue University's horticulture dept. says:
Several herbicides can be used to eradicate poison ivy.
Amino triazole sold under the name Amitrol, is highly
effective and safe to use. Available as a liquid or wettable
powder, it should be applied during periods of rapid
plant growth to insure maximum kill. Thorough vegetative
coverage is essential and repeated applications may
be needed. The herbicide glyphosate (Roundup,
Kleenup) is also effective in eradicating the pest. A
combination of dicamba (Banvel) and 2,4-D can be used
to advantage where herbicide drift is not a factor. However,
2,4-D should not be applied in locations where
other sensitive species grow in close proximity to poison
ivy. Read and understand instructions on the herbicide
label before making applications.
One thing to note. Herbicides that are translocated such as 2,4-D must be used at the strength on the instructions. If you increase the strength, then you will kill the leaves before the poison can be translocated to the roots. This is a common problem with some herbicides leading people to think that the chemical didn't work.
Since poison ivy has oily leaves, be sure to use a spreader-sticker agent. This will help it penetrate the oily surface to be absorbed by the leaves.
Used to be that many herbicides were dissolved in diesel, rather than water. Worked really well when I did that. Probably not legal anymore. Don't know if it will work with glyphosate.