In theory, its only value would be in topical application to reduce chances of infection on the skin which may break  open due to the ivy blisters.

Poison ivy/oak/sumac produce an oily liquid called urushiol which is the irritant to the skin. Colloidal silver won't have much effect on that substance, but will help prevent skin infections that could follow exposure.

If you know you've been exposed, it's a good idea to wash the area as soon as possible using some kind of detergent (like dishwashing liquid) and water... that will work to break up the plant oil.

There are other remedies for posion ivy that work better than colloidal/ionic silver.  The silver may help disinfect and help heal but it will do little to stop the poison ivy in its tracks and might even spread it if you aren't careful and let the silver solution drain on to other unaffected areas of the skin.

If you have a problem with poison ivy or poison oak, you may find some helpful suggestions here:

http://www.tbyil.com/Poison_Ivy_and_Poison_Oak.htm