flair(French) intuition, perspicacity, instinctive understanding. The word is, of course, also an English expression, but I am assuming that Joyce's use of italics implies that this is a foreign term. Walter Pater, whose style is being parodied in this section, was fond of including numerous italicized foreign words in his prose, especially in his early work.
piazzetta(Italian) a small veranda. Properly, piazzetta means a small square in a city, being a diminutive of piazza. The latter, however, can refer to the veranda of a house, which seems to be what is required here.
alles vergängliche(German) all that is transitory. These are the opening words of the final chorus in Goethe's drama Faust, Part II (1832):