The two recommendations differ because of the weed killer used. Davidg is recommending glyphosate while I am recommending a broadleaf weed killer. Even glyphosate works best if there is some vegetation left on which to apply the spray.
If the entire area is infested with poison ivy then go with glyphosate. If the poison ivy is in patches and you would like to keep any grass that is mixed in with it, stick with a broadleaf killer.
