Only if Bruno Heller didn't lie about the clues he gives in interviews it must be Kirkland:
#1. Red John is someone on the list of 7. Bruno Heller said we can trust the list.
#2. Red John has been there from the very beginning. He said we will realize that he has been there from the beginning. Partridge was in season 1 episode 1, and Kirkland was in the 'prequel' episode where Jane starts at the CBI.
#3. Partridge is dead, Bruno Heller confirmed the death after the last episode, that eliminates him from the above.
#4. Red John is someone we've seen before. We've seen Kirkland many times obviously.

The only thing that's baffling is that he also said that he knew who Red John is from the start, and even though Kirkland was there according to the story, he was not in season one, and that season was originally written as a stand alone story.