Translation:You are not pretty, I am pretty!
Hi KzRau. It's a question of emphasis. 'Tha mi brèagha' is a bald statement: 'I am pretty.' But 'Tha mise brèagha' is a countervailing statement heavy with implications. 'You? You think you're pretty, with a face like a torn scone? Ha ha! You're having a laugh! You're not pretty! Sheesh! I (Tha mise) am pretty!'