McNamara has the distinction of being the first person to "disappear," leaving Dagny in a serious lurch business-wise. He was a contractor who was going to help Dagny with her railroad in Colorado. He plays an important role in terms of the book's ongoing mysteries; he disappears right after Eddie mentions his importance to his mystery friend, whom we later find out is John Galt. This coincidence raises a definite red flag and alerts us to the possibility that there is more to Eddie's friend than meets the eye. McNamara effectively helps heighten the tension in the ensuing scenes between Eddie and his friend.
In Atlantis, McNamara is one the people who is somewhat blunt with Dagny:
"You know, Miss Taggart, when it comes to the John Galt Lines – maybe it's I who've followed it and you who're betraying it." (188.8.131.521)
This pointed comment forces Dagny to reconsider her decisions and raises a lot of questions about the strike in general.