Dysbiosis of the gut microbiome has been correlated with irritable bowel syndrome (IBS). Fecal microbiota transplantation (FMT) is being explored as a therapeutic option. Little is known of the mechanisms of engraftment of microbes following FMT and whether the engraftment of certain microbes correlate with clinical improvement in IBS. Microbiome data, from a previously reported placebo-controlled trial of treatment of IBS with FMT or placebo capsules, were used to investigate microbial engraftment 15 days, 1, 3 and 6 months after treatment through assessment of gains, losses and changes in abundance of amplicon sequence variants (ASVs) and microbial diversity (CHAO-1 richness) between the FMT group and the placebo group. These data were compared to changes in IBS Symptom Severity Scores (IBS-SSS). Twelve days of treatment with 25 daily multi-donor FMT capsules induced significant short- and long-term changes in the recipients' microbiomes for at least 6 months, with persistent engraftment of a variety of anaerobic bacteria from keystone genera, such as and and increased microbial diversity, particularly in patients with low initial diversity. FMT recipients lost ASVs after treatment, which was seen to a much lesser extent in the placebo group. No ASVs increased to a greater extent between FMT responders and non-responders following treatment. Major long-term changes, lasting for at least 6 months, in the gut microbiomes of IBS patients are seen following treatment with FMT capsules. None of these changes correlated with clinical improvement. The relationship between the microbiome and the etiology of IBS still remains unsolved.