Ireland Reaching Out posted a very interesting article, "Ten Facts about Irish Gravestones."

Here's one point that grabbed my attention:

Fact #3: Headstones were rare in Ireland until the mid-19th century.

While some headstones dating back to the 17th century can be found in Ireland today, upright headstones were rare before the 19th century. 

• The oldest marker you would most likely find would be a gravestone -- a full-lenth slab laid flush to the ground (to prevent the dead from rising again).
• Headstones for early 19th-century ancestors that appear in excellent condition are likely to be modern replacements erected by a later generation.
• A legacy from overseas often resulted in headstones being erected long after the fact, and this can result in errors such as date mismatch.

The Ireland XO article is definitely worthy of a full read. Check it out here.