Ironhaul Trail is but one of many trading routes that snake through Haven's wilderness to link travelers to Lethian's Crossing. Its most notable feature is a seasonally-accessible river crossing that is prone to flooding during the high summer months.
Points of interest
Near the center of the map you'll find Aster and Will. The latter is trying to extort the former.
Raetommon is nearby and Will will request that you go talk to him, otherwise you can't cross. Feel free to sic Barik on him.
Whether Raetommon allows you passage through the Trail depends on your allegiance; if you're on the Disfavored or Anarchist paths, you'll have to fight your way through.
There is a hidden cache (Subterfuge check) in the top-middle of the map, after crossing the bridge.