Answer:
A 12MHz ±0.25% parallel resonant crystal such as the HCM49-12.000MABJ-UT HCM49 12.000MABJ-UT CITIZEN FINEDEVICE, Crystal, 12 MHz, SMD, 11.4mm x 4.65mm, 50 ppm, 18 pF, 30 ppm, HCM49 | Farnell
Ireland can be connected externally to the internal oscillator circuit of MAX3421E IC via the XI and XO pins.