You can also warp to the entrance if you've activated the Survival Guide at the entrance.
You will need a Lamian Fang Key (or a Thief) to open an Iron Gate inside Arrapago Reef. You may farm this key either in Caedarva Mire or Arrapago Reef or can obtain one by checking a ??? in Caedarva Mire (H/I-7), on a small ledge behind a Jnun. A character can obtain a free Lamian Fang Key once per Vana'diel day (resets at 0:00 game time) and they can be sent between characters on the same PlayOnline account. [Note: you will technically need TWO Lamian Fang Keys (or use the other option); one to flag and one to "finish"].
You do not need a Lamian Fang Key to get to the ???. Using the Ilrusi Atoll Staging Point will get you to Map 2. From there, go to point A to get to Map 1. Then go to the tunnels using point m. That puts you at the same spot as if you used the Lamian Fang Key with the method above. Exit using the lower left Iron Gate to get to the ???. (Note: There are Soulflayers along this route that are True Hearing. There are also Heraldic Imps in the cave right next to the staging point that are True Sight. Going this way is NOT ideal for your level 40 Corsair unless you have reraise.)
Note: If you had not previously received the final cutscene from Ratihb after finishing Luck of the Draw, you will need to complete that cutscene first, then speak with Ratihb again to obtain the Trump Gun.