Ilminster is a market town and civil parish in Somerset, England, with a population of 5,808. Bypassed in 1988, the town now lies just east of …Ilminster is a market town and civil parish in Somerset, England, with a population of 5,808. Bypassed in 1988, the town now lies just east of the junction of the A303 and the A358. The parish includes the hamlet of Sea.