Will Brash kill ironweed

No, Brash does not kill ironweed.

Specifically, the herbicide in question can be found under the product name Brash or Weedmaster. It contains the active ingredients dicamba and 2,4-D amine. Under the best circumstances, it controls ironweed (Vernonia spp) by about 75%. It demands repeated applications since the purple-flowered, tall perennial often responds with 75+ regrowth.

A more effective combination is an herbicide whose active ingredients include 2,4-D and triclopyr.