The irony is in the juxtaposition of normally pleasant words with words that are very unpleasant.
Irony is when something is unexpected. You would expect a festival to be fun and games, and you would expect a carnival to be similarly harmless. In this case, though, the words are used to describe a lot of something—bad things.
It is the season of oppression, dark metal, and tears.
It is the festival of guns, the carnival of misery
The word “festival” is contradicted by “guns” and the word “carnival” is contradicted by “misery.”
The dark time is not a time of peace. It is a time when you are a target. While you are sleeping, the enemy is coming for you.