The last episode does not mean that Kirkland is RJ.  From Red Dawn we know that Kirkland has watched the RJ case from a distance since the beginning.  Why is he suddenly getting involved?  What changed?  Simple answer:  Lorelei was arrested.  Kirkland does seem to have connections with RJ but it's entirely possible that Kirkland is involved because of his own connections to Lorelei, Miranda, and Lennon.  Kirkland only seems interested in the killings connected with Lorelei.  He didn't show up for The Red Barn episode because his interest isn't in RJ but in the Lorelei mess.

Let's consider some possibilities:

Kirkland knows RJ and asked him to kill Miranda for personal reasons.  Similar to Red John's Footsteps, RJ got one sister and killed the other for Kirkland.  Kirkland interferes with Jane's investigation and kills Lennon to hide his involvement.

RJ knows about a terrible crime Kirkland committed.  Therefore, Kirkland kills RJ's friends when they surface just in case RJ told them about Kirkland's crime.  That could be why Kirkland asked if Lennon recognized him - to see if RJ had told him.  Kirkland gets in Jane's way because he knows that if Jane ever finds RJ, then RJ might tell him about Kirkland.

Kirkland is RJ's main friend.  The one that killed Rebecca and now Lennon.  He'll be the focus of the season final as Jane tries to capture or interrogate him.  He asked Lennon if he knew him to see if Lennon recognized him from eight or so years ago when Lennon brought Miranda to RJ and Kirkland was presumably there.

I really do not think that Kirkland is RJ.  Please add other plausible theories explaining Kirkland's role.