merry drinker; happy drunk
Describes a person who becomes cheerful and laughs a lot when drinking alcohol.
He's a merry drinker; whenever he drinks, he's always laughing happily.
person who laughs easily
Refers to someone who tends to laugh readily, not necessarily related to alcohol.
彼女は笑い上戸で、ちょっとしたことでもすぐに笑い出す。
She laughs easily; even the smallest thing makes her burst out laughing.