Mirthful, a.
1. Full of mirth or merriment; merry; as, mirthful children.
2. Indicating or inspiring mirth; as, a mirthful face. Mirthful, comic shows. Shak. -- Mirth"ful*ly, adv. -- Mirth"ful*ness, n.
Shades of meaning
Fernald has no entry of its own for mirthful; he lists it under happy.
He lists it without comment; the entries above explain its neighbors.