Antonyms of unhappily :
blithely, gayly, happily, jubilantly, merrily, mirthfully, with happiness, cheerfully, cheerily, delightedly, gaily, gleefully, good-naturedly, lightheartedly, rejoicingly, sunnily, blithesomely, calmly, casually, dispassionately, easily, impassively, indifferently, lightly, nonchalantly, stoically, unconcernedly, favorably, well, blissfully, gladly, joyfully, joyously.
Examples of usage:
Unhappily, very few," replied Polnivski. "Stories and Pictures", Isaac Loeb Peretz.
Then, my noble friend, geometry will draw the soul towards truth, and create the spirit of philosophy, and raise up that which is now unhappily allowed to fall down. "Plato's Republic", Plato.
This was unhappily not the case. "Complete State of the Union Addresses from 1790 to the Present", Various.