There would have been 1125 people travelling on each day on the free Xmas flights, multiply that by 4 and you get 4500.
Yup, that much is clear. Ryanair fly 737-800s, which have 189 single-class seats. 189 multiplied by 24 flights would be 4,536 seats.
EI-A330-300 is correct 4500 covers both departure and arrival.
That's not so clear. 4,500 seats would depart, and then those same 4,500 seats would return and "arrive". That looks like 9,000.