I believe you'll find Poison Ivy more susceptible to herbicide when first
leafing out in springtime and when storing nutrients to its root system in
autumn. Meantime you can spend exasperating time spraying and re-spraying
during other periods with no satidfying results in the long run.
The reasoning behind this approach is that the roots are expending stored
energy to sprout leaves in Spring so the leaves will in turn feed the
roots; reducing the leaf structure interrupts the feeding cycle, so to
speak. Similarly, the leaves in Autumn are more likely to transmit the
herbicide to the root system before closing shop for the season, and the
herbicide has all winter to do its work on the root system.
Timing is important for the best results. I've used RoundUp Ultra (mixing
from concentrate) for about fifteen or twenty years or longer and it's very
effective when used properly.