Here is a quote from my little Book of Kells, a nifty hardback pocket-sized edition published by Random House UK with text compiled by Ben Mackworth-Praed. This is from the commentary on Plate IV (The Evangelical Symbols).

"The Man is for St. Matthew, in recognition of his emphasis on the human side of the Saviour. The Lion represents St. Mark who stressed Christ's power and royalty. The Ox or calf stands for St. Luke, a sacrificial victim in token of his emphasis on Christ's priesthood. The Eagle is for St. John, the Evangelist who soars to Heaven, as St. Augustine puts it, and gazes on the light of immutable truth with keen and undazzled eyes. Symbols, often pagan in origin, played an important role in the life of the early and medieval Church. Among them are the fish, an acronym in Greek for Jesus Christ, the peacock for immortality (see Plate VII) and the stag for the soul thirsting for redemption."