what figure of speech is showing in the paragraph as Samantha left for the party a car splash her with mud and ruined her new dress. What a great day I’m having she Muttered

The figure of speech being used in this sentence is irony. This is because Samantha says "What a great day I'm having" sarcastically after her dress is ruined by a passing car. The use of irony emphasizes the contrast between her words and the unfortunate situation she is in.