。
She got dressed up and went off to the party.
Literal
She [topic-は] dressed-up [and-て] party [to-に] headed-out went.
出かけて行った chains two verbs: 出かける (to head out) + 行く (to go), with the ~ていく pattern adding a sense of departure from the speaker's location. She dressed up, headed out, and went away (to the party). 盛装して is the te-form of 盛装する, connecting the dressing-up action to the departing action.