L'Isle sur la Sorgue is serviced by both trains and buses from Avignon and by trains from Marseilles should you fly into that particular airport.
Whilst it is a short distance from Avignon to L'Isle, the trains are not all that frequent and the last train from Avignon departs at about 7.15pm. In the middle of the day there are very few buses or trains in or out of L'Isle. So if you are planning on having dinner in Avignon, be sure to bear in mind the train times. The last bus leaves Avignon at 7.30pm to the best of my knowledge.
Likewise the trains to and from Marseilles are few and far between as well, but if you get your times right, you can do all sorts of day trips within the time frame of the public transport system.
In l'Isle-sur-la-Sorgue, parking space is limited. We were there on June 12th and could park almost easily but later, in full season, it must be difficult to find a parking space close to the city center. When there is the Antics fair, I can hardly imagine the situation ! Better to come by bus!