There is a bus that goes directly from the train station to Sirmione. (This is the bus that goes to Verona) The bus goes from the train station and then drives almost to the gate of the old city of Sirmione. Sirmione itself is also the whole peninsula. Automobile traffic in the old city is very limited. You can expect aprox. 500m walk or so from the bus stop to the hotel. The price is a few euros, Im not sure exactly what the price is as I have only taken this bus on my way to Verona. It should be around 2-3 euros pr. person, I would estimate.
When you arrive at Desenzano train station, you could also ask the taxi drivers that are outside how much they would charge to drive you directly to the hotel. It is approx. 10km from Desenzano to Sirmione (old city).
Hope this helps.
Check these maps here:
maps.google.com/maps/ms?oe=utf-8&cli...00476b16d93930ccc596
maps.google.com/maps/ms?oe=utf-8&cli...00476b1a5e5b6f294ecf
