Does anyone know if roundup poison ivy killer will hurt my established hellabores or climbing hydrangeas.?

If it is close or growing in those plants I do not think I would risk it. If you do try a paintbrush instead of spraying to make sure does not touch them.

The product used glyphosate and triclopyr to kill. Both of these chemicals are at least as lethal for hellebores and climbing hydrangeas as it is for poison ivy.

I have a Round-up sprayer that has a cone on the end of the spray wand. The cone comes down completely over the stuff I want to spray, protecting the surrounding foilage. I did miss once and thought I caught a clematis stem, but I ripped it off the plant right away and no harm done. I'm a lot more careful now, and carry hairclips in my tool bag to get such things out of the way.