Some facts and figures
Based on a Ryanair 737-800 not sure how the Mango aircraft are configured all economy or some business also.
Basic empty wt = 42500 Kg
189 Pax @ 93 Kg = 17577 Kg
ZFW = 60077 Kg
737 800 MLW = 65300 Kg
Trip to BFN = 2500 Kg one hour flight +/-
Take off max = 67800 Kg
Max fuel = 7733 Kg
Fuel for Cpt to Bfn back to Cpt then George as alternate would require about 8800 Kgs
I am guessing the crew took as much as they could but for all the reasons already mentioned in this thread, they found themselves with a bit too much.
Short sector flying is always prone to catching you out on the landing weight limitation.
Happy flying.