As I see it, "마침내" is used in positive or negative sentences in the meaning of "finally", but "드디어" is only used when something we've been waiting for or expected finally has happened as we wish. \^o^/

드디어: 무엇으로 말미암아 그 결과로. 마침내: 드디어 마지막에는 비슷한 말: 비로소, 끝내, 결국

비슷한것 같은데요.. 흠.. it feels like very very same word to me.. there's no difference. both words mean "eventually, finally .. "..