If you know the dates you where there you should probably put them on the application.
Problem is that Schengen was established in 1995 (20 years) ago so since that time you have already changed your passport more then likely a couple of times so even if you mention it you should be prepared to prove that these actually took place especially if these were done prior to SIS, which I think would be prior to 1990.
Long story short what the immigration authorities are trying to establish is that you will go away once the term of your visa or maximum duration of stay has been reached. So if you have 5-6 entries and exits during the Schengen era they may be sufficient but if you feel like adding a list of all of your entries and exits put them on a separate sheet and attach to the application if possible.