Could someone point me in the right direction for finding a marriage certificate for Catherine Fitzgerald (1824 Ireland) & Daniel Costigan (1819 Ireland) who were married on December 25, 1850 by a Roman Catholic priest in Cardiff, South Wales. This couple left within 2 years of their marriage for the U.S. where Daniel fought in the civil war. I got the marriage information from the civil war pension record that Catherine filled out. Any help would be most appreciated.

Is there a possibility that it was Catherine's second marriage? The only one that seems to fit is:

1851 Qtr1 - Costigan, Daniel (Cardiff 24 498)
1851 Qtr1 - BURK, Catherine (Cardiff 24 498)

I guess a second marriage is a possibility. And since the marriage took place late in 1850 it wasn't recorded until the first quarter of 1851. Where would be the best place for me to order a copy of this marriage record?
Thank you very much

The easiest option is to order on-line at the GRO:

http://www.gro.gov.uk/gro/content/certificates/default.asp