“The Schengen Agreement” refers to two agreement in 1985 and 1990 dealing with the removal of border controls between countries, under European Union law.
The agreements also covered cross-border police and judicial co-operation, which some coutries signed up to ( ie the UK) without agreeing to remove physical borders.
Ireland is however signed up to the Schengen agreement on removal of physical borders.

For our information here, we are only interested in those countries within the border control agreement.

The effect on flights and for passengers booking in is that a flight between two Schengen countries is likely to be treated the same as a domestic flight. In some cases, flights to Schengen countries (and domestic flights) will be from a separate terminal to International flights.

The countries coloured olive on the map below are in the Schengen agreement for border controls.