Is onnellinen related to onnea?
Yes, it all comes from onni, happiness or luck. It's also a male given name.
Onnea is the partitive of onni.
Is there a difference between "iloinen" and "onnellinen"?
Yes, onnellinen means a deeper and longer lasting satisfaction with life, iloinen means that you are in a good mood. Happy can mean either, since we don't have any context here.