The irony of rules
The last rule is a reminder that these rules are advise for young monks, not for experienced transcendentalists. By pursuing union with creation and God through their intense appetite for thought, poetry, meditation, and discipline, any monk can quickly see that rules aren't actually necessary. That's the greatest irony of the book: It's a book of rules about how to eventually move beyond rules.
The irony of independent enlightment
Jesus might have prayed in his closet, or in the deserts of Israel, but he was not just an ascetic mystic (he definitely is that, among other things). Instead of seeing a loner, Jesus's demonstration for monkhood is to be embedded in community. Even a mysterious man such as Jesus himself belongs to a community. Ironically, independent enlightenment seems only attainable through lived community with others.
The irony of sacredness
This is not intended by its author as sacred scripture. And yet, because the rules were helpful to many young monks transitioning into life at the monastery, it has become a true staple in communities that strive for enlightenment. In the rules, sacredness doesn't just apply to the scriptures. It applies to other people too. It applies to nature, and it applies to the sound of silence in meditation. Everything is sacred, but in a normal way.
The irony of injury
To receive a wound from another can be seen from this ironic point of view: isn't is secretly God on the other side of all monastic strife? Certainly, the question must be asked (by a monk especially), why does God allow these horrific instances of disappointment, betrayal, dishonesty, and even true injury? The irony of injury is that although it is caused by the one who injures, it is also caused by God, allowing a monk to learn ironically from unfair suffering.
The irony of humility
The monks abide by the principles of Christian doctrine, especially the sacrosanct nature of human experience, the divinity of the soul, and the quest for union with the creator of the universe—and yet, their rules are simply to obey true humility, many of them. The rules are deference and apology, and strict obedience to discipline. Why? Because of Christ's ironic example of humility. Christians believe Jesus was literally God, so why live a life of service and suffering for others who are hateful and broken? That is a transcendental question, it seems.
