Roundup won’t generally kill established poison ivy/oak/sumac.... you need to find brush killer. If it’s an area where you don’t care if anything grows there in the next year or so, you can use a broader-spectrum vegetation killer, such as Ortho(?) GroundClear, which will stay in the soil, sterilizing it for ~12 months. Most weed and brush killers work only within the plant, and become inert upon contact with the soil.... that’s why you are supposed to use them on actively growing, leafed-out weeds or brush — because the chemical is absorbed thru the surface area of the leaves.
Use a coarse spray and make sure you thoroughly wet as much of the poison ivy/oak/sumac plant as possible..... don’t skimp! Also, plan on having to reapply the chemical to areas of the plant that become exposed after some of it dies back. It’s tenacious, but it can be conquered.... it’s just not a one-and-done application.