Are any of the old St. Mary's County newspapers on the internet?

Yes. The "Chronicling America: Historic American Newspapers" site at the Library of Congress has the following:

St. Mary's beacon 1845-1863: 

http://chroniclingamerica.loc.gov/lccn/sn89060119/

St. Mary's gazette 1863-1867:

http://chroniclingamerica.loc.gov/lccn/sn89060120/

Saint Mary's beacon 1867-1921:

http://chroniclingamerica.loc.gov/lccn/sn82006687/


The St. Mary's Public Library site at 

http://stmalib.archivalweb.com/reelSelector.php

has the following:

Saint Mary's beacon. (Leonard Town, Md.) 1923-1984

The Enterprise 1952- to 3 months ago