。
What a stylish hat you're wearing!
Literal
Fashionable honorable-hat [object-を] honorable-wear-[be-doing-でいらっしゃいますね].
Triple-stacked keigo: お帽子 (お + noun), お召しになる (honorific 'to wear,' from 召す), and いらっしゃいます (honorific いる/ある). 洒落た is the past-participle of 洒落る meaning 'stylish, chic' as a modifier. This is textbook sonkeigo — the speaker elevates the listener's action of wearing a hat through multiple layers of politeness. ね at the end softens it into an admiring observation.