The objective of this paper is to examine measurement issues and data problems in the analysis of trends in male mortality differentials by socio-economic group (SEG) in Ireland between 1981 and 1991. The study is based on mortality data supplied by the Central Statistics Office and population data taken from the 1981 and 1991 Census. The recording of the occupation of decedents worsened between 1981 and 1991, making it impossible to discuss trends in mortality differentials by SEG in Ireland with any confidence. Significantly more deceased people ended up in the residual “unknown” occupational category in 1991 than in 1981. This is related to an increasing problem of apportioning “gainfully employed” decedents to a socio-economic group rather than to any problem with people described as “not gainfully employed”. This is all we can say at the moment with respect to the analysis of trends in mortality by 12-category SEG in this country.