my flight is arriving at 1:30 am. can i still take the free shuttle bus to royal plaza hotel?

Well, according to your arrival time, there is no free shuttle bus for you to take.
So upon landing, you need to take bus no.N23 at Passenger Terminal Building, Cheong Tat Road, get off at Argyle Street Playground, Ma Tau Chung Road. Then transfer to bus no.61 at No. 216 Argyle Street and go to Mongkok East Station. Then walk northeast around 280 yards.
The whole way takes around 75 mins with the fare of 26.5 HKD per person.