A formal education is not necessary to be a good historian. David Irving may not have completed any kind of higher education but that was because of financial constraints, not an academic failure on his part. So no, you don't have to disregard him as a scholar because he doesn't have a college degree.

No, there are far better reasons to reject Irving. His complete disregard for academic standards, his blantantly obvious bias, his callous disregard for truth and quite overt holocaust denial however? Those are very good reasons not to take David Irving seriously in any scholary context.

You'll find a more complete thread here by /u/commiespaceinvader and /u/kieslowskifan among others.