Sedimentary rocks are formed when layers of sediment build up on top of each other and compact under their own weight. Limestone is made of compacted fossils, Sandstone is made of compacted sand, an Mudstone is made of compacted soil.
Igneous rocks are formed when lava seeps up from the mantle of the earth and cools. These rocks cool into crystal shapes. If they cool quickly, the rocks form into large crystals like the basalt in the Giants Causeway. If the rocks cool slowly, they will contain small crystals, like the granite found in the Wicklow mountains.
During the collision of continental plates, Igneous and Sedimentary rocks can be subjected to great heat and pressure. This then changes them into other, harder, stronger rock types known as Metamorphic rocks. For example, the Sedimentary Mudstone could change under much heat and pressure into Shale, Slate, then Schist. The tectonic collision could also fuse different rock types together. For example, Schist and Mica could form to create Mica-Schist, which is abundant in the Wicklow mountains.
This photo shows the relationship between the three different rock types.
Photo from http://www.nj.nrcs.usda.gov/partnerships/envirothon/soils/images/rockcycle.gif
