2. The ancient high crosses were not grave markers. They were probably boundary markers, and might have marked the boundaries of monasteries. That's likely why they were so tall--so they could be seen from a long way off. In more recent times Celtic crosses have been used as grave markers. Here is one in NYC.
3. The crosses were carved from sandstone or granite. The high cross at Clonmacnoise was carved from a single slab of sandstone.
4. The crosses, especially those with biblical scenes, might have colorfully painted. They were stone versions of illuminated manuscripts.
5. The crosses with biblical scenes were probably used as teaching tools for folks who could not read or had no access to scripture, which was everyone not residing in a monastery or somehow connected with a church or a king.
Celtic By Design posted an informative article on Celtic crosses that you should check out.