Trained as a computer scientist, Gallagher combines his understanding of statistical analysis with his interests in politics.
Gallagher created the Gallagher index, a least squares index of proportional representation that measures an electoral system’s disproportionality between votes received and seats allotted in a legislature.
Gallagher is the author, co-author or editor of eighteen books, including The Politics of Electoral Systems (2005, co-edited with Paul Mitchell), Politics in the Republic of Ireland (4th ed., 2005, co-edited with John Coakley), and Representative Government in Modern Europe (4th ed., 2006, co-edited with Michael Laver and Peter Mair), which are standard textbooks in their fields.
He has also written around 70 journal articles and book chapters (his most cited independent work is "Proportionality, Disproportionality and Electoral Systems" in Electoral Studies [1991]), and serves on the editorial boards of various journals in the discipline, including Electoral Studies, European Journal of Political Research, Representation, Party Politics and Irish Political Studies.
Gallagher was president of the Political Studies Association of Ireland, from 1994 to 1996.