Presumably the Devonport in Devon, UK, the same as Devonport, Tasmania.

In Devonport, Tasmania the population is roughly 25,000

Devonport is part of the city of PLymouth and a major naval dockyard on the southwest coast of Great Britain, in the county of Devon. There is also the city of Devonport (named, as many Tasmanian towns are, for British towns and people) on the north coast of Tasmania, the island which is the southernmost state of Australia. Devonport in Tasmania is the port for the ferry which runs between the mainland and Tasmania.

The British Devonport is at the mouth of the River Tamar: Tasmania also has a River Tamar not far from Devonport but the city itself is at the mouth of the River Mersey.

It docks in Devonport

Quite simply, it is far more straightforward for the Spirit of Tasmania to dock in Devonport. It is a shorter journey straight across Bass Strait than for the ferry to go south, then negotiate its way through the harbour to Hobart. Taking a car or hiring one in Devonport is a more direct and cost-effective option.