I believe Kirkland is the head of 'Red John,' a cult group of 'subconciously hypnotised' people, brainwashed to carry out his bidding. Theres a long running theme of mind control and this is key to solving the riddle.
Patrick has known Kirkland for most of his life but doesnt realise it, because Kirkland isnt who he appears to be... literally! .... Kirkland has undergone facial plastic surgery to change his identity and is the reason why Lennon doesnt recognise him. The clue is in the trademark smiley face :)
Also, i believe Kirkland actually grew up with Patrick, both from traveller roots, until something happened between them. Kirkland has a list of deciples, all of which are unaware theyre working for 'red John,' until a 'trigger' reveals the truth and a 'Red John' disciple is revealed. Heres the catch... Patrick Jane is the 'trigger' and is unaware hes working for the 'Red John' as well.... weve never seen the crime scene of patricks murdered family and theres a reason for that :)