Mullaly specific:
Andrew J. Morris Genealogy, listing of O'MULLALLY / LALLY
SurnameWeb Registry Page - Mullaly Surname Resource Center
Descendants of Alexander Gilliland - with a reference to Rupert Mullaly
Mullally Mailing list - Instructions on joining

Message boards:
Mullaly Family Genealogy Forum at GenForums
FamilyHistory.com Mullaly Message Board

Genealogy Resources:
Hamrick Software - U.S. Surname Distribution - Enter a surname into the form and you'll get a map of the United States showing the distribution of people with this surname within the 50 United States.
Cyndi's List of Genealogy Sites on the Internet - Over 58,300 links, and she wrote a book too. Whew.
The Genealogy Home Page - Sponsored by some people who make family tree software. Another collection of links but well kept up.
Treasure Maps-The How-to Genealogy WebSite - a good site for a how-to in beginning.
FamilyHistory.com - hosts message boards for surnames.
Genealogy.com - you would think this would be the obvious site. A mostly pay site, from Family Tree Maker Software. Very comprehensive.
RootsWeb.com - site trying to keep genealogy info free to all.
FamilySearch Internet Genealogy Service - sponsored by the Mormon church, a great repository of information.
Irish Migration Resource Center
RootsWeb Obituaries Center
Genealogy Searching Center - A surname search engine jump-off point.
Ellis Island Records - The American Family Immigration History Center records from Ellis Island in New York, a main port for immigrants for over a century.