At least one of the marble columns in the parish church in Kilcullen has a series of somewhat regularly-spaced white patches. I've often wondered whether they were the vertebrae of the back bone of some prehistoric marine animal. Marble is a metamorphic rock, formed when limestone is "cooked" and melted by heat from magma within the Earth. Limestone itself is a sedimentary rock, formed when marine organisms die and their skeletons or shells settle to the bottom of the ocean. Over tens or hundreds of millions of years of geological time, the deposits build and the layers are compressed into rock by the immense pressures.
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The most interesting fossil I've found was from about 6 feet deep when I was helping to dig a hole near Kilmeague. The lump of red rock was embedded with cockle shells, presumably dating back hundreds of millions of years to an era when Ireland, or rather the land fragments that would become the island, were located south of the equator.
University College Cork are running a project at the moment called the Great Irish Fossil Hunt, lasting from the 2nd April to 31st August. They're asking the public to be on the lookout for what might be fossils, some of them even hidden in the fabric of buildings. Photos can then be sent to the UCC team. More info here.