Complementarity is one of the key concepts underlying the spatial development strategies introduced on the island of Ireland a decade ago. While neither Northern Ireland's Regional Development Strategy nor Ireland's National Spatial Strategy defines the concept explicitly, both documents suggest that it relates to differences in functional roles between places, thereby linking complementarity to the concept of sectoral specialisation. Using data on employment by industrial group from the respective censuses of population, this paper examines the extent to which urban centres in Ireland displayed complementary patterns of specialisation at the regional level at the time the spatial strategies were introduced. The analysis finds little evidence of this, revealing instead a strong tendency towards similar specialisations of neighbouring centres, including those in cross-border settings. The findings point to the need for a more nuanced specification of the policy precept of inter-urban complementarity that is sensitive to both sectoral and geographical scale.