Installing shelf over block construction walls, plus more
Your 1 by firring strips should not be randomly spaced - they should be 16 inch on center - just like studs. There is a set distance you have from sheetrock to concrete block (about 1 inch total). There might be electrical run behind the sheet rock as well. You might want to invest in a tool that you might be able see behind the sheet rock. 

All of that said : 

You will need to anchor into the concrete block via through the fir studs.

Issues:

Concrete block is not hollow everywhere so the anchor you will need is a metal style plug - much like a lead lug.

Concrete block has hollow area (which might be filled with concrete) depending on what year your house was built (if after 1997 you can be assured it should be every 24 inches I believe and there you will also find a strap down rod so the roof can be strapped to the foundation every 24 inches). 

So in these non-hollow areas again a concrete plug style anchor will be needed. 

In a hollow area you can use a butterfly anchor. 

You should find areas of the wall where you have those stud fir strips - because as you tighten the bolts/screws holding your heavy iron against the wall it will compress the wall and also there will be 'free play' that could allow the screw/bolts to sag from the weight - loosening the hold into the concrete and also damaging your wall. To exaggerate what I am saying imagine a 10 foot pole extending out from that wall and some one standing on the end of it - it will sag and act as a crow bar on the anchor point.