Seems like they are trying to inform/ describe.
This is because they are telling you when and what. It also seems to be a real event.
So I guess,
"To inform that In May of 1911, thousands of people lined up in Belfast, Ireland, to watch the launch of the Titanic."
Answer: The authors purpose is to inform you about how popular I guess the Titanic was when it was ready to launch.
Explanation: My explanation for this is that he includes the amount of people who were there to watch the Titanic launch.
