Unless you plan on either #1) digging up all the irises and moving them to a sunnier spot or #2) digging up all the irises, adding compost to the soil and replanting in the same space- then there isn't much else you can do for them since they do best with more sun and soil that drains well. As far as what to
plant behind them, you could try Japanese anemone, Foxglove, Astilbe, Monkshood, Goat's Beard, Peach-leaf Bellflower, Milky Bellflower, Turtlehead, Bugbane, Lobelia, Ligularia, and/or Senecio to name a few.