My 2¢ only, but yes, the tachometer will work on the Iron. The tach itself has five electrical wires coming in and all five also connect to the speedo (at least the wiring diagram in the '07 Dyna Service Manual shows this.) By tapping into the wires you could get the tach operating.
However, I think the incompatibility here is the mounting. The Iron speedo mount, like on my Dyna, isn't set up for a tach. You'll need something like on the XL Roadsters of a few years ago or a mix-or-match combo like those from Yankee Engineunity as shown in the Dennis Kirk catalog.
Suggest you check the wiring diagram in the Sportster Service Manual (or for the '09 Dyna you have) to be sure, and go from there.
An option, and the one I'm looking at, is the Dakota Digital speedo/tach combo , MCL-3204, with blue or red lighting. A bit more than the HD one you are looking at, but it fits into the existing mount and is a direct electrical plug-in.
Good luck, and let us know what you come up with (plus pictures).
Silver