Irility
A shorthand term to describe a state of being old to the point of senility (thus unable to reason), and irritable (usually as a result of being unreasonable). Irritability + Senility = Irility

Also, "Irilitable" as an adjective.
"That old fat hobo over there was yelling at the wall, looks like a bad case of irility".
"The old man was very irilitable"