The molecular epidemiology of group B Streptococcus (GBS) in Ireland was investigated. Invasive (n = 132) and non-invasive (n = 45) isolates, collected in 2007-2011, were analysed by multilocus locus sequence typing, capsular polysaccharide (CPS) serotyping, profiling of surface proteins, pilus islands (PI), and antimicrobial susceptibility. Isolates grouped into 45 sequence types and five main clonal complexes (CC). CC1, CC17 and CC23 represented 67.2 % of isolates and the most prevalent serotypes Ia, III and V. Serotype and surface protein genes were largely predictive of CC. Accordingly, CPS V/alp3, CPS Ib/CPS II/bca + bac, and CPS Ia/eps predominated in CC1, CC12 and CC23, respectively, and CPS III/rib in CC17 and CC19. Supporting their vaccine potential, all isolates harboured at least one PI, of which the PI-1 + PI-2a combination was most prevalent. Macrolide resistance was found in 18.6 % of isolates. erm(B) and the globally disseminated CC1/CPS V were the most common resistance mechanism and CC/CPS type, respectively. CC17, significantly associated with neonatal disease, was also prevalent in pregnant adults, but was underrepresented among non-pregnant adults. Two of 46 CC17 isolates (typically CPS III) were CPS IV. Sequence analysis confirmed capsular switching and their relatedness to CC17/CPS IV strains recently characterized in France. CPS IV, detected only in invasive isolates (6.8 %), was most prevalent in adults (12 %) and showed an increase in prevalence to that reported (1.4 %) for invasive isolates in Ireland 1997-1999. Increases in serotype IV and evidence of capsular switching in CC17 highlights the importance of ongoing surveillance of GBS and may have implications for vaccine development strategies.