Give one example of how Irish Monks preserved Christianity and one example of how they preserved knowledge in general.
The major way the Irish monks preserved knowledge in general was by copying books. Back in those days, the only way to produce a book was to copy it by hand. The books by the great classical scholars of Greek and Roman times were being lost, but the Irish monks had copies of some of them and made more copies. By doing this, they can be said to have preserved knowledge.
During the Middle Ages, lots of pagan tribes were attacking and conquering what had been the (Christian) Roman Empire. So you can say Christianity was under threat. But the Irish monks kept Christianity alive and later helped (as missionaries) to spread the religion back into Europe. But it's probably overstating things to say that Christianity would have died without them.
During the Germanic invasions, much scholarship of the classical world was lost. Also some Christians did not think too highly of classical authors, who were deemed immoral for their scurrilous verses. For example, Catullus, the Roman poet exists only through one manuscript. What the Irish monks did was copy a lot of manuscripts and preserved them. This was an invaluable work for us all, because apart from these monk and some others, we would not have access the one of the truly great periods of human civilization. Imagine if we did not have the great historians such as Livy, Tacitus and Polybius or the great playwrights like Sopholces and Aristophanes? We would be poorer.