in a happy way, or in a way that makes you feel happy:
Sylvia laughed merrily.
quickly and in a pleasant way:
The fire soon began to burn merrily.
The clock ticked merrily in the corner.
not thinking about possible problems that might happen as a result of what you are doing - used to show disapproval [= blithely]:
Meanwhile, the company is merrily pushing ahead with its plans.