Transport for 5pax from Mong Kok to HKIA. My flight departs at 01h30.

Generally speaking, you need to arrive the airport at least 2 hours in advance.
Thus from Mongkok to HKIA, you can take bus no.E21 at Fife Street, Reclamation Street, and get off at Passenger Terminal Building, Cheong Tat Road directly. The whole way takes around 75 mins. And the ticket fare is 14 HKD per person.

From TST to HKIA, take bus no.A21 from Haiphong Road, Nathan Road and arrive at the airport directly. The duration is about 65 mins with the fare of 33 HKD per person.