Abstract Background Despite the public health implications of language difficulties associated with social disadvantage, there is a dearth of effectiveness studies investigating the effects of targeted speech and language programmes in this area. Aims To determine the effects of a targeted selective community-based child language intervention programme (Happy Talk), which simultaneously engaged with parents and early childhood educators, in the Republic of Ireland. Methods & Procedures A mixed methods methodology was applied with quantitative outcome and qualitative process data collected. Effectiveness was examined using a quasi-experimental single blind study design comparing Happy Talk with ‘usual care’ across four preschools. Qualitative process data were also gathered to examine the acceptability and feasibility of the Happy Talk approach in practice, and to identify factors to improve the probability of successful wider implementation. Child language (PLS-5) and quality-of-life measures were administered pre- and immediately post- the 11-week intervention. Responsiveness was assessed as the parental outcome, and the oral language environment of preschools was measured using the Communication Supporting Classroom Observation Tool (CSCOT). Retrospective acceptability was analysed with reference to the theoretical framework of acceptability (v 2). Outcomes & Results Pre-/post-expressive and composite language scores were collected for 58 children, and receptive scores for 54 children. Multiple linear regression revealed significant intervention effects for comprehension and total language with large and moderate effect sizes, respectively (0.60 and 0.46 SD). No significant effect was shown for parental responsiveness. No effects were found for the preschool environment or children's quality of life. Preschool staff deemed the programme to be an acceptable method of enhancing children's speech and language skills and rated the intervention positively. Conclusions & Implications The Happy Talk pilot effectiveness trial shows that comprehension can be improved (with a large effect) in preschool children from areas of social disadvantage, following an 11-week intervention, in which parents and preschool staff are simultaneously engaged. The ecological validity of the programme, as well as feasibility and acceptability to staff, make it a suitable programme to be delivered at scale. What this paper adds What is already known on the subject Up to 50% of children from socially disadvantaged areas enter preschool with speech and language difficulties. The majority of intervention studies are (1) researcher led; (2) efficacy trials carried out in ideal conditions; and (3) focus on working with parents or early childhood educators rather than engaging with both groups simultaneously. Many studies omit child language outcomes, and those that include them tend to show relatively modest effects for expressive language and negligible effects for receptive language. What this paper adds to existing knowledge This pilot study shows that the Happy Talk programme, which is embedded in the community and which simultaneously engages with parents and early childhood educators, is highly effective in improving children's receptive language skills. These findings are particularly important in the context of (1) the study taking place in real world conditions; and (2) the programme being designed and refined by speech and language therapy services, rather than one which is researcher led. What are the potential or actual clinical implications of this work? Implementing an 11-week targeted selective community-based language intervention can result in a large positive effect on receptive language for children from areas of social disadvantage. The study findings highlight the importance of embedding intervention programmes in the community and of simultaneously engaging with parents and preschool staff.