We've got blooming ivy growing around a tree. Since several weeks lots of hornets have been whirring in the foliage and I wonder whether they want to feed on insects that are lured by the blossoms or whether they come for anything they get from the ivy plant.
Hornets, just like wasp in general, hunt insects to feed their larvae but feed themselves with carbohydrates like nectar (or lemonade). So it is quite probable that the hornets are feeding on the nectar of your ivy. wikipedia