The cross in Irish Soda Bread is a form of scoring which creates a weakness in the surface so the bread can expand as it bakes. If you don't score it, you could get some bursting out in other places, such as the bottom and sides, ruining the appearance of your bread. It also allows the heat to penetrate into the deepest part of the loaf more easily, ensuring it cooks quickly and evenly. Functional purpose aside though, legend has it that the cross was cut to allow the devil to escape while it baked. Also, being a Christian country, the cross is thought to symbolize the crucifix. By cutting the cross you are giving thanks. The cross also helps the "breaking of the bread" into 4 easy pieces once it's baked.