Here's an
interesting article on this topic here on Aikiweb.
The late Ken Cottier, one of the Western pioneers at Aikikai Hombu Dojo in the 1960s, used to say that iriminage was the one technique that Ueshiba O-Sensei created himself.
But then, the modern form of iriminage was argued by Stan Pranin to be a creation of Kisshomaru Nidai Doshu.
Alex