Titanic was built in Belfast Ireland at the Harland and Wolff ship yard.

I guess you could say that the main designer of the Titanic would be Thomas Andrews. He went on board Titanic and did not survive the sinking. He was last seen in the smokeing room staring at a painting of Plymoth Harbor (as seen in the new movie if you have seen it).

Early in Titanic's construction, Alexander Carlisle was the chief designer, but aparently he became frustrated from the project and left it.

There were other designers and navel architects. Two names that come to mind are Edward Wilding and Roderick Chisolm.