There are a number of very good sites around which offer good explanations of the various musical modes, but I wasn't able to answer a couple of basic questions which relate to guitar chords.

First Situation

This happens in tunes like Ragtime Annie and Acrobat's Hornpipe, or in a song like The Stone Outside Dan Murphy's Door. Let's say that the main key is G major ... then the key effectively changes to D major, before wandering back very cleverly to G major. Is this music in a particular Mode? Or is there a convention for telling a guitarist that the tune changes to D type chords in lieu of G?

Second Situation

Scottish tunes and songs are often modal with successive chords moving a full tone, eg Am to G, or A to G. Is this typically a particular mode?

Any help from qualified practitioners would be appreciated.

Regards
John