"Mother" and the Betrayal of the Technology (Situational Irony)
A central irony of the film is that while the technology is advanced enough to allow for interstellar travel and futuristic amenities, when it comes to the danger posed by the alien, the technology cannot help the crew members. The crew members put a great deal of faith in Mother, and ask it to help them solve issues, but it only leads to their downfall in the end. Even though it has the name of "Mother," the system is revealed to be responsible for creating a terrifying reality in which almost all of the crew dies and in which she/it is powerless to help in any tangible way. Whereas most mothers seek the safety of their children, Alien's "Mother" ensures their death, betraying them.
Ripley's Survival (Situational Irony)
Ash regards the Alien as a pure organism, bent on survival and seemingly programmed to kill without remorse. Ash explicitly names the alien's primary virtue as its ability to survive. Given its genetic pre-disposition to prioritize survival, it becomes markedly ironic that Ripley outlives it. While the remorseless, blood-thirsty, and thoughtlessly violent creature seems bound to be able to survive, it is the conscionable human who survives. Thus, the viewer sees that the true survivor is Ripley, and that human ingenuity wins out over the ruthless alien in the end.
The Unexpected Transmission (Situational Irony)
The plot of the story is initiated when the ship is rerouted towards a potential distress call. The crew members are hesitant, but they are willing to do their job and follow the transmission, because they believe it is their obligation to help. However, it is soon revealed that their interpretation of the signal as an S.O.S. is a mistake, and the signal is actually a warning. In an instance of situational irony, the people who had imagined themselves to be rescuers become the people in need of rescue, when they welcome an unfriendly alien aboard their ship.
