We were in Montserrat in 1994 a year before the volcano blew. In fact a very popular hike back then was to a beautiful waterfall through the tropical jungle at the base of the volcano. Now all of that is wiped out, including the capital city of Plymouth. It is eerie seeing photos of the ruins of Plymouth buried under a pyroclastic flow knowing that we were walking those very same streets.
The southern half of the island which was devastated by the eruptions is an exclusion zone and remains as such. Tourists are typically not permitted in the exclusion zone as the volcano, although quiet for the past couple of years, remains active and dangerous. When it erupts it spews deadly pyroclastic flows that you cannot outrun.
You can get to Montserrat from neighboring Antigua but if you're going in the hope of seeing volcano activity you will likely be disappointed. A better bet is to plan a trip to the big island of Hawaii when Kilauea is erupting. This will be much closer to what you probably have in your mind's eye.