The NHC forecast tracked showed a eyewall hit on Cuba, basically the Cuban Keys. But their discussion said they thought that would not weaken Irma. They were wrong about that even though their predicted track was correct.
It went just a few miles further south over Cuba than they predicted - and then the shear and dry air from the Gulf took a big toll on the south side of Irma just after it passed the Keys. It was a very narrow avoidance of the feared Cat 5 hit.
That depends on which NHC forecast you look at. They were moving the track on and off Cuba, several times. On the Thurday, when they first started talking about a Cat 4/5 hitting Miami, they were showing it brushing the coast of Cuba, but NOT making landfall there... hence, little to no weakening.
http://www.nhc.noaa.gov/archive/2017/IRMA_graphics.php?product=5day_cone_with_line_and_wind