The Nomadic is called the last White Star Line ship in existence. It was a tender for the Titanic and her sister ship Olympic. A tender ferries "passengers, their baggage, mail and ship's supplies to and from large ocean liners moored offshore." (that's from Wikipedia)

The Nomadic has a long, interesting history. During its heyday it played host to nobility, celebrities, and the rich and famous. It was pressed into service during both world wars. Eventually it became a floating restaurant permanently moored in the Seine River in Paris. It was almost scrapped several times. In 2006, the Northern Ireland government Department for Social Development bought it at auction (I think it was the only bidder), and it was brought back to Belfast for restoration, and is now open to the public.
(By the way, the building to left of the Nomadic in this photo is the Titanic museum.)