There is some information, including directions, about Taverna Agni here:
I think your options are going to be your hire car or a taxi. It's about a 45 minute drive. I don't know if buses run from Paleo in the evenings but I do know that they don't run up the north east coast (the Kassiopi route) in the evening.
I do appreciate that you may not want to spend this much time away from Paleo but if you really want to be able to have a drink at Agni you might consider booking one night's accommodation in one of the neighbouring resorts (e.g. Nissaki or Kalami). You've got the hire car anyway so you can make your way over during the day. This would probably cost a similar amount to a taxi there and back and you would also be able to experience the Agni boat taxi.
Alternatively you could drive over for lunch, you could incorporate it into a day on the east coast or you could spend the day at Agni (sunbeds and small motorboats are available for hire).
Have a great holiday, Deedee