Dear Allison
According to a website (
http://www.ajmorris.com/dig/imr/surn.htm), Madigan is a variant for O'Madden. For further information the site refers you to "Irish Families, Their Names, Arms and Origins," a book available from amazon.com.
I am also curious of the meaning of Madigan. If you could please tell me the outcomes of your research, I will be very happy.
all the best