Dennis Gahagan (c. 1817–) was an early settler in San Diego, California.
Dennis Gahagan was born c. 1817 in Ireland. Ireland (pronounced /ˈaɾlənd/ Éire) is the third largest island in Europe, and the twentieth-largest island in the world
He came to the U. S. and joined the Army, which was recruiting for the Mexican-American War. The United States Army is a military organization whose primary mission is to "provide necessary forces and capabilities. He came to San Diego by 1849 as a soldier with the First U. S. Artillery (Magruder's Battery). He and his fellow troops helped protect the countryside during the war.
In 1849 Gahagan was elected Judge of the 1st Instance of the District of San Diego. From 1849 to 1850 Gahagan was an Alcalde (Mayor) of San Diego. This is a list of pre-statehood Mayors of San Diego California.