Thinking of mandolin as upside guitar (assuming you use eadgbe tuning of course) will ultimately end up causing more confusion ...

In Irish music in particular, the mandolin is used almost exclusively as a melody instrument. We don't use chop chords in the way, for example, that bluegrass players do. And so we need to think of it as an instrument in its own right and develop the "finger memory" required to get around the two or three scales used in Irish music (essentially D, G, A and the minor relatives thereof).