A relevant free and open source competitor: https://github.com/EasyTier/Easytier
Again, the difference is: tailscale/easytier/wireguard creates a VPN network on your computer/phone/whatever. So all apps can benefit of it
iroh is a library . This is for developers who want to integrate this P2P routing logic inside their app. It is closer to bittorrent than it is to tailscale.
Both approaches are great but fulfill completely different needs.