McGrath said "The point is, people who deny the holocaust are liars, not real historians."

Well, I don't think you oppose a point of view by immediately denouncing your opponents as liars. These people are very likely deluded, but they may fully believe what they're saying.

There is in fact a body of work that supports the revisionist view of the Holocaust. A search online will reveal numerous references to the "holocaust Myth", including one from Pravda, the official Russian periodical.

The crux of the revisionist argument boils down to the following, as far as I can tell:
1) The wholesale murder of Jews in German camps was never state policy, and there is no written and signed command by Hitler which states this.
2) It is true that Jews were unwanted in Germany, but the program was one of deportation, not slaughter, with many Jews being sent to the USSR, etc
3) The gas chambers were never used for execution. Zyklon B found in the camps was a standard pesticide used in the German Army for delousing.
4) The figure of 6 million deaths is grossly exaggerated. An example is shown with Auschwitz. Initial estimates of dead were in the 1-1 1/2 million at Aushwitz, but were later revised to 300-400,000, but without any corresponding adjustment in the total 6 million figure.

On and on, and, well, you get the picture. My point is, these people are not just denying that anything happened. They are fairly specific, with points that I believe need to be addressed in argument. We can never assume that Truth can be defended by simply shouting "Lies!" We sometimes must do battle on the field of rational discussion.
On CNN, Glen Beck said "the Iranian President is trying to figure out if the Holcaust happened? Here's a clue...rent Schindler's List."
That kind of argument is idiotic. A movie can be many things, moving, transcendent, revelatory...but it isn't necessarily true. Watch "Triumph of the Will."