This paper presents the findings of a multi-country assessment of irregular migration stocks, that is, the assessment of the estimated number of irregular migrants living various countries at a given point in time. The analysis covered the period 2008 to 2023 and was undertaken as part of the Measuring Irregular Migration and related Policies (MIrreM) project, which looks at irregular migration in 20 countries. MIrreM is a follow-up to the Clandestino project, which carried out a similar exercise between 2000 and 2008. Some of the key findings of the report include: Based on the most recent estimates in the Database, there were between 2.6 million and 3.2 million estimated irregular migrants living in 12 European countries (Austria, Belgium, Finland, France, Germany, Greece, Ireland, Italy, Netherlands, Poland, Spain and the UK) over the period 2016 and 2023. This figure represents less than 1% of the total population and between 8% and 10% of the population that was born in countries outside of the Schengen Area (for EU countries) and the Common Travel Area (for Ireland and the UK). Amongst the countries studied by the MIrreM project, the United States has the largest estimated irregular migration population not only in terms of absolute numbers, but relative to its total and foreign-born populations Finland, meanwhile, has the smallest estimated irregular migration population in terms of its size and its share of the total and foreign-born populations amongst the countries we covered. Taking the 2008 estimates produced under the Clandestino project as a baseline, there was no definitive change in the estimated number of irregular migrants across the 12 European countries.[1] At the individual country level, however, many of the estimated irregular migration populations in the countries covered by the project appear to have changed: in three countries, estimates suggest greater numbers, in five countries, the estimated irregular migration population remained the same and, in five countries, it declined. [1] That is, the new estimate’s lower bound is higher than the 2008 estimate and its upper bound is lower; the updated estimate is more finessed but there is still a significant range which remains a numerical black box, within which we cannot say with any confidence whether there was an increase, decrease or no change in the aggregate estimate.