Joe Messenger of Morgantown West Virginia tied the first Irresistible, first called the Deer Hair Drake and later renamed back in the 1930's.  Many variations of the Irresistible have appeared since messenger tied the first fly many years ago. This one is a "Wulff" varient with a yellow tail.

A true all-purpose dry fly, designed to float well in fast broken water and catch everything from Bluegills to Steelhead.

Because they float so well, Irresistibles, and Humpy patterns are the choice of many anglers to use as the lead fly in two fly indicator rigs to fish very small flies effectively. 

The larger sizes  are popular  as a skating  fly for summer run  steelhead.