Help me calculate spectral irradiance of an LED from a LED spec sheet
Well it's nice you have a link to the LED.  You can see all the data you need on the second page.  (The good thing about an IR led is you don't have to deal with lumens :^) You can see the total power is 40 mW. And the radiant intensity is only 170 mW/sr at 100 mA of current.  (you've got to pulse it quickly to get more.)  

You can also see that the beam (full) width is 20 degree's (+/-10 deg.)
One way to think of steradians is as square degrees.  This is OK
So a radian is about 57 degrees, and a square radian (Steradian) is about (57)^2 degree's squared.  So as a check with a total power of 40 mW over (20 deg)^2 is about 40 mW *(57/20)^2 = ~325 mW/sr.  That's about twice the listed number... which is fine because the width is the half power point and the intensity is a measure of just the central maximum.  (the 40 mW is not constant over the beam profile so my quick calculation over estimates the intensity.)