The 1912 McTigers: Red McDermott, Dan McGarvey, Jim McGarr, Deacon McGuire, and Pat McGehee
The 1908 McRed Sox: Amby McConnell, Jim McHale, Ed McFarland, Deacon McGuire (manager), and Doc McMahon
The 1902 McOrioles: Dan McGann, Herm McFarland, John McGraw, Sport McAllister, and Joe McGinnity
What baffles me is that Deacon McGuire and John McGraw also managed their teams (McGuire managed the Red Sox, not the Tigers). Personally I never knew that guys used to manage and play at the same time, but it's certainly interesting that they did. Talk about McMulti-Tasking.
Anyways, those are the only three instances in which five McPlayers played on the same team in the same year. The Pirates will at least tie that record this year, and they will probably break it. Andrew McCutchen, James McDonald, Casey McGehee, and Nate McLouth are sure bets to make the team out of spring training, and Michael McKenry is a pretty good bet as well. If Daniel McCutchen makes an appearance (which he probably will) or Kyle McPherson makes his debut this year, the Pirates will be in a McClass of their own.
And I started all of this when there was just one Mc on the team.