The irony of biological life
It seems perfectly obvious that the only life that could exist in the universe is animal life. However, the novella introduces a terrifying possibility, that perhaps robotic lifeforms could develop, should the computers decide to self-replicate, which is where we find ourselves in this plot, surrounded by a type of life that ironic to even conceive.
The irony of nuclear warfare
The absolute worst thing that could happen is offered in the novella as the best recourse. In other words, the threat of self-replicating robotic life would be so severe, that our best option would be the same option we'd worked so hard to avoid during the Cold War, when the novella was written.
The irony of space travel
The implication of space travel has always been a good thing, especially in the 1950's and 1960's when this story was invented, when space travel literally defined the geopolitical landscape of the earth. Therefore, the idea that traveling to an unknown world could be bad was deeply ironic when the story was published, although it may seem less obvious now.
The irony of swarm mentality and individualism
Another serious political issue during the time the story was written was that of communism, socialism and capitalism. In the story, we are shown an alien life form whose power comes from their ability to self-organize and operate as a swarm. That could be seen as a contrasting viewpoint of the individualism of the west. It's ironic that the source of the alien's power is their uncanny ability to get along.
The irony of the title
By calling the ship, and also the novella, The Invincible, Lem introduces an ironic element, the fact that actually, no, humans are not invincible, and something as simple as robot insects could put them on the brink of disaster.
