McManus was the name of a resident of Caldos colony who had passed away some time prior to 2370.
His gravestone was in the same cemetery where Felisa Howard was laid to rest. In that year, the area around McManus's grave was the scene of a battle between Felisa's granddaughter Beverly Crusher and an anaphasic lifeform named Ronin. (TNG: "Sub Rosa")
This character was only mentioned in writing.
The gravestones bore in-joke references to various names; this may have also been meant as an in-joke.