Recent weeks have offered New York theatergoers a variety of seasonal delights, from an exuberant Broadway revival of the musical “Elf” to a new staging of “The Dead, 1904,” Irish Repertory Theater’s elegant, moving adaptation of the James Joyce story. While the latter production is still running at the American Irish Historical Society, the company is now also serving a scrumptious confection at its off-Broadway home, this one inspired by the work of a Welsh scribe.