According to dean :
But poison ivy is a vine.

Yes it is. But the stems aren't "hairy". Not hairy at all.

Larger/older ones have woody stems.

It has the three leaves (a pair and then a single in front).

Well, so do Trilliums ;-)

If the three leaves aren't (usually) symmetrical (so you wouldn't
say "a pair and a single"), it's probably not PI.

[Generally speaking the three leaves are on stems from the same
node - the leaf stems are all the same length and start from
the same place.]

"Leaves of three" is simple to remember, but there are many things
with three leaves that _aren't_ Poison Ivy (even if symmetrical).
Secondly, leaf shape on Poison Ivy varies a lot - more than
descriptions or even pictures of it would suggest.

I recognize it by having had one extremely knowledgeable person point
out a "guaranteed" PI plant. From then on I'm going on overall impressions
of leaf colour/size/sheen/veining/margin, plant/leaf/stem proportions & colour.

I don't think I've been wrong yet.

We're a little too far north for it to "vine" appreciably. Yeah!

This is an untouched forest so I guess they have been
growing there a while. What else can it be? Its all over the forest
floor too.

I can't think of something specific (that could be called a three leaf
vine), but there are others.
--
Chris Lewis, Una confibula non set est
It's not just anyone who gets a Starship Cruiser class named after them.