Some topiary frames use smaller wires to hold the shape of the animal. This allows the moss to be held by the frame alone and for soilless mixes to fill in the cavity. In your case, the shape is created by a core leaving minimal or no room for the soil... To know how it was put together, you may need to pry it apart... But my guess is that this wasn't meant for long term planting. Possibly only for seasonal use. If you wish to revive it, get some moss sheets available from garden centres, take your fishing wire and start wrapping the frame with it -- while doing so, take a soilless mix (add water retention agents) and then replant your ivy as you go along tying it down with your fish wire. You will need several plants to fill this up. However, I would strongly suggest looking for alternate plants that don't produce a lot of roots and a very tolerant to dry conditions.

Hope that helps.