If Conor McGregor has a weakness, it's probably his ground game. While his wrestling and Jiu-Jitsu is certainly not "weak," he does seem to struggle the most against opponents looking to take the fight off the feet.
It's worth noting as well that both of his two defeats came by way of submission (and both in the first round as well, so fatigue was likely not an issue).
This supposed hole in McGregor's game will almost certainly be tested in his upcoming lightweight title fight against Rafael dos Anjos. Dos Anjos has seven submission wins on his record (including a Submission of the Night award), and is a Brazilian Jiu-Jitsu black belt.
McGregor seems well aware of this though, and is reportedly working very hard on his grappling ahead of his March match-up. He recently uploaded this video of him training his guard passes with SBG Ireland's Peter Queally.
For a longer look at McGregor's grappling training, here's an older video showing off the wrestling practice he puts in. This video was shot prior to McGregor's most recent win over Jose Aldo.
UFC 197 will take place on March 5, 2016.