Background: Universal screening using the International Association of Diabetes and Pregnancy Study Groups (IADPSG) criteria has identified a prevalence of gestational diabetes mellitus (GDM) of 12.4% in women living in Ireland. Women with prior GDM are at increased risk of developing type 2 diabetes later in life. A number of risk factors linked to the development of type 2 diabetes are potentially modifiable through lifestyle and behaviour changes, and medical management. No previous Irish studies have adequately investigated the efficacy of lifestyle intervention programmes in reducing these risk factors in women with prior GDM. Through a two-group, parallel randomised controlled trial (RCT), this study aims to assess the clinical impact, cost-effectiveness and psychological experience of the Croi MyAction intensive lifestyle modification programme for women with prior GDM.Methods/Design: A total of 54 women with a history of GDM and persistent post-partum glucose dysfunction (impaired glucose tolerance (IGT) or impaired fasting glucose (IFG)), are randomly assigned to a control arm (n = 27) or to the Croi MyAction intervention group (n = 27). The control arm receives usual health care advice - written information on diet and lifestyle changes for reducing diabetes risks and visits with general practitioners as required. The intervention group receives usual health care as per the control group in addition to attending a 12-week intensive lifestyle modification programme known as Croi MyAction. Croi MyAction involves 2.5 hour sessions once per week (for 12 weeks) comprising a group exercise programme, group health promotion or education seminars, and one-to-one meetings with a multidisciplinary health care team to personalise risk factor reductions. Randomisation and allocation to the intervention arms is carried out by an independent researcher, ensuring that the allocation sequence is concealed from study researchers until the interventions are assigned. The primary analysis is based on glucose dysfunction, comparing a mean reduction in fasting plasma glucose (FPG) levels on a 75 gram oral glucose tolerance test (OGTT) in the two groups at a one-year, post-intervention follow-up. The trial is funded by the Irish Health Research Board (HRB). Ethics approval was obtained on 27 March 2012 from the Clinical Research Ethics Committee, Galway University Hospitals, Health Service Executive of Ireland (Ref: C.A. 691).