Not an expert but this is how I understand it. Yggdrasil is a P2P mesh network. You configure peers to join the network and your computer becomes a relay node for everyone else to use. It doesn't work behind a NAT without port forwarding.

Iroh is kinda just a connection protocol. If you get given a public key for another computer, you can establish a connection. Like you would an IP address. The magic is in being able to establish that connection regardless of where either device is, and keeping that connection alive through changing network conditions.