If you have failed the mission trying to defeat Irena, you will have to reload the checkpoint. When you do so, it will spawn you in a hallway. There should be a door to your right and another a short distance down the hallway. Use the door that is on your right when you spawn. When you open that door, the computer that needs to be hack will be directly across the room from you. Kill all of the terrorists, but do not assign a team member to hack the computer. Face the back of the computer, not the controls, and turn right. You will see a set of stairs. Walk up those stairs very slowly (try crouching), and you should be able to ease your way past Irena without her seeing you. When you pass all of the glass windows, you will see a metal door. Make your way to that door and "Saving Game" will appear. Once that has happened, just run back past Irena and it will cause you to fail the mission. However, when you reload checkpoint, the metal door will be open for you.