It is on the border of counties Roscommon (province of Connacht) and Westmeath (province of Leinster). Although the River Shannon forms the historic border between Roscommon and Westmeath, the Local Government Act of 1898 designated all of Athlone (Urban) as belonging to Westmeath, including areas west of the river. Much recent growth has occurred outside the official town boundaries. Monksland for example, a suburb on the west side of the town, is not within the official town boundaries, yet it is the most populous area of County Roscommon.
The Census of Ireland 2011 recorded the population of the town at 20,153 with in excess of 40,000 thought to be living within and outside of the official town boundary.