Iringa is a good starting point for a visit to Ruaha National Park. Transport and accommodation arrangements can be made through Iringa Info, which is opposite Hasty Tasty. You are looking at around US$200 per day to hire a 4×4 to get there and drive round the park — this includes a driver and the cost of the fuel.
Ask at Iringa info for cars but they are a bit pricy. Neem crafts is a good spot to pick up info on independent smaller operators and also local tourist gems like guided walks and the newly found rock paintings. Tatanca offer a very good service. their office is on the Dodoma Road heading away from town.
Sumry, Sutco, and Upendo have morning departures to Dar Es Salam around 8AM. Scandinavian Express does not go any longer, despite what their internet site indicates. Instead Taqwa and Falcon do so and arrive from Dar to Iringa around 1PM. Those buses do not come to Iringa bus station but roadside. Normally they overnight at the border.
Heading in the other direction Chaula bus run the best service towards Mbeya, with 2 buses leaving every morning.