The difference is Greece have a large number of players playing first team football every season in the Champions League and Europa League for Panathainaikos, Olympiakos, PAOK etc.
Plus you have some of the top greek players playing first team football for big teams in other leagues.
I saw PAOK playing Shamrock Rovers in Tallaght and they were very impressive with very strong, very fast football. They looked alot stronger than some of the big other teams like Partisan Belgrade, FC Copenhagen who recently played in Tallaght and had lots of internationals in their sides.
Ireland by comparison have very few players playing regular top European football and most of the our squad are just premiership squad players who are in and out of their first teams.