This is the symbol of the early Celtic Goddess Brigid and her christianized counterpart Brigit. Brigid is the Goddess of poetry, healing, and craftsmen, especially metalworkers. She was by far one of the most popular goddesses in prechristian Ireland and the surrounding islands. Her name is still associated with several towns and place names, as well as countless sacred wells and holy sites.
Most people do not realize that there are actually several methods of creating the symbols know collectively as Brigid's Crosses, all were originally constructed of grain or straw. The cross pictured in the center is the most commonly known variation.
The earliest version of this "cross", pictured above to the left, invariable predates Christianity itself. It is constructed with three arms reminiscent of the triskelion and another reminder of the Celtic fascination with the number three. This form is almost never encountered today, save in the Gaeltacht and some very rural parts of Ireland.
The cross to the right is interesting, it seems to have been found in and around County Donegal around the end of the 18th century, according to period travel books call it a Brigit's Cross. Though this is often repeated, I am not certain this is an accurate conclusion, as English guide books of that era are known to contain a great many inaccuracies on Eire. Though it is possible that this is actually a variant of the Brigit's cross specific to that region, as there are a great many variations of this concept in Ireland.