Ireland has the unique, and somewhat paradoxical distinction in modern times, of giving the status of First Official Language to a Celtic Language which, at the same time, is designated a lesser-used language of the European Community. This state policy gives national status to the Irish-languages forms of the current placenames of Ireland and is officially supported by a government-appointed advisory Commission, An Coimisiún Logainmneacha. In the days of total British administration in Ireland the Ordnance Survey, that is the government mapping agency, through its vigorous Topographic Department in the early 19th century devoted great care to the Celtic toponymy of Ireland mainly for the purpose of providing written forms which would be suitable for use in the English language. This paper describes the part presently being played by the national mapping agency of modern Ireland in implementing the state policy of bilingualism. In particular, reference is made to the use of the science of toponymy in restoring the original Irish forms of the placenames, thus providing rich sources for linguistic, dialectal and historic studies.