Background Treating severe emotional dysregulation and co-occurring substance misuse is challenging. Dialectical behaviour therapy (DBT) is a comprehensive and evidence-based treatment for borderline personality disorder (BPD). It has been hypothesised that the skills training, which is a facet of the full DBT programme, might be effective for people with severe emotional dysregulation and other co-occurring conditions, but who do not meet the criteria for BPD. However, there is limited research on standalone DBT skills training for people with substance misuse and emotional dysregulation. Methods A mixed methods study employing an explanatory sequential design was conducted where participants with a dual diagnosis (n = 64) were recruited from a community-based public addiction treatment service in Ireland between March 2015 and January 2018. DBT therapists screened potential participants against the study eligibility criteria. Quantitative self-report measures examining emotion regulation, mindfulness, adaptive and maladaptive coping responses including substance misuse, and qualitative feedback from participants were collected. Quantitative data were summarised by their mean and standard deviation and multilevel linear mixed effects models were used to estimate the mean change from baseline to post-intervention and the 6-month follow-up period. Thematic analysis was used to analyse the qualitative data. Results Quantitative results indicated reductions in binge drinking and use of Class A, B and C drug use from pre-intervention (T1) to the 6-month follow-up (T3). Additionally, significant improvements were noted for mindfulness practice and DBT skills use from T1 to T3 (p < 0.001). There were also significant reductions in dysfunctional coping and emotional dysregulation from T1 to T3 (p < 0.001). Significant differences were identified from pre to post intervention in reported substance use, p = 0.002. However, there were no significant differences between pre-intervention and 6-month follow up reports of substance use or at post-intervention to 6 month follow up. Qualitative findings indicated three superordinate themes in relation to participants' experiences of a DBT skills training programme, adapted from standard DBT: (1) new lease of life; (2) need for continued formal aftercare and (3) programme improvements. Participants described reductions in substance misuse, while having increased confidence to use the DBT skills they had learned in the programme to deal with difficult emotions and life stressors. Conclusions This DBT skills training programme, adapted from standard DBT, showed positive results for participants and appears effective in treating people with co-occurring disorders. Qualitative results of this mixed methods study corroborate the quantitative results indicating that the experiences of participants have been positive. The study indicates that a DBT skills programme may provide a useful therapeutic approach to managing co-occurring symptoms.