Your IRO has an English 68mm BB, the most common type of BB shell out there today. What kind of bike did the Ultegra crank/BB come out of? Chances it is also 68mm English.

Depending on what kind of frame your Ultegra crank came out of your swap should work, but.....
assuming your bike is in stock single speed mode with a 42 mm chain line you'll have to use the inner chain ring on the Ultegra crank which typically has a 41mm chain line. To have similar gearing you'll have to get a larger chain ring or smaller rear gear to get the same gearing. This is assuming you have 39t inner chain ring on the Ultegras and a 42 to 46t fron chain wheel on your cranks now.
If your stock single speed cranks have a 130 BCD (bolt circle diameter) you could have the possibility of swaping your chain ring off the current crank on to the inner position of the Ultegra crank.