！
Happy birthday, Muiriel!
Literal
Honorific-birthday congratulations Muiriel!
The standard birthday greeting. お is the bikago (beautifying) honorific prefix that softens 誕生日, and おめでとう (literally 'auspicious') is the plain form of the congratulatory expression — おめでとうございます is the polite version. Together they form a fixed greeting; no verb, no particles, just a vocative.