Recognising the gaps in current migration statistics, the study proposes this novel approach as a means to offer more accurate and nuanced indicators of irregular migration. Addressing the existing challenges related to international migration data, such as its incompleteness, lack of recency, and the need for harmonization, the paper asserts that mortality data can provide valuable insights into these issues, further shedding light on the demographics, living conditions, and health access of undocumented migrants. Using Belgium as a case study, the research demonstrates how death registry data can be employed to develop migration indicators. By applying this method, we can better identify and understand the characteristics of unregistered populations, offering age- and gender-specific results that could significantly inform future policy decisions. By investigating the mortality paradox among migrants, the paper reveals fascinating trends that offer deeper insights into the factors influencing migrant mortality. The study posits that the mortality extrapolation methodology could serve as a crucial tool to fill in the knowledge gaps in migration statistics, and hence, a valuable source of harmonised statistics on irregular populations in Europe.