Is my dad's condition of having irregular heartbeat still curable? Does he need to visit doctor often? We don't know how he's been since his last check up was more than few years ago already.

Arrhythmias doesn't necessarily need treatment except only if it can cause severe symptoms like blood clots etc. There are different cures for arrhythmia, depending on what your father has. Check ups may be regularly required in order to make sure of how he really is doing--if it's being well-regulated or starting to develop risks like stroke and other heart disease.