St Brigit's Cross is nearly always presented as being a four-armed cross like this:
http://www.scottishtartans.org/stbrigit.jpg
However, it was originally a three armed cross representing the Irish triple goddess, of which Brigid was one aspect.
It was originally related to the corn-dollies and made at harvest time to give thanks to Brigid for the fertility she provided, hence it still being made of straw today.