To specify this further: when arriving at the airport, go to the S-Bahn counter and ask for one "
Isar Card Wochenkarte Ringe 1-4" (
IsarCard for one week, i.e. from Monday to next Monday noon; rings 1-4 means valid within Munich city borders). Will be EUR 15,80 and is good for as many trips as you like, any time, with any public transport within Munich city borders. Does not have to be validated in one of the blue boxes, as the duration of validity will already be printed on it.
Also ask for 3 "Tageskarte Aussenraum" (ticket for one day, outer zone), as both the airport and Dachau are not within Munich city borders. So, one for getting from the airport into the city, one for your Dachau day trip, and one to get you back to the airport. One will be EUR 5,00.
Important: as the Tageskarte Aussenraum is only valid for the area outside Munich city limits, only the combined tickets (Tageskarte and IsarCard) will be good for the airport and Dachau trips. Also, be sure to validate the Tageskarten in one of the blue boxes at the station entrances of U-Bahn / S-Bahn before entering the train. The blue box will print the day of validation on the Tageskarte, enabling the fare inspectors to see that the ticket is valid for this or that day.