Marry is a verb meaning (1) to become someone’s husband or wife, and (2) to officiate at a marriage ceremony. The adjective meaning jolly or festive is spelled merry, with an e.
Merry works as a noun in the verb phrase make merry, meaning to be festive or to celebrate, and it also appears in the compound noun merry-go-round.
Though they are pronounced identically in some parts of the English-speaking world, the words are not related. Marry came to English in the 14th century from French and ultimately Latin sources.1 Merry has Germanic origins and has been in English in various forms for as long as the language has existed,2 but its modern spelling wasn’t settled until the 18th century.3
While the contents of the first letter remained in her mind, she was all surprise—all astonishment that Wickham should marry a girl whom it was impossible he could marry for money. [Pride and Prejudice, Jane Austen (1813)]
Rose had been in high spirits, too, and they had walked on, in merry conversation, until they had far exceeded their ordinary bounds. [Oliver Twist, Charles Dickens (1838)]
Thus in Michigan it is provided that no man shall marry his mother, grandmother, daughter, granddaughter, stepmother, grandfather’s wife, son’s wife, grandson’s wife, wife’s mother, wife’s grandmother, wife’s daughter, wife’s granddaughter, nor his sister, brother’s daughter, sister’s daughter, father’s sister, or mother’s sister. [Parsons' hand-book of forms (1884)]
Sometimes the Boosters’ lunches were given over to speeches; sometimes they were merry and noisy; and when they were noisy Candee was the noisiest. [Harper's Magazine (1920)]
Now she is being tipped as the girl most likely to marry Prince Charles and become the next Queen of England. [Sydney Morning Herald (1974)]
Hinduism, Christianity, Islam, Buddhism – the more the merrier, except nothing seems to make him truly merry. [Herald Scotland (2012)]