it is probably meant to be in the Dorian mode as that and the myolydian were generally the resources from which melodies were derived in western folk music
That has never been true at any time, and certainly wasn't true in Ireland or Irish America in the middle of the 19th century
And yet the Original Poster seems to have found that the swallowtail Jig tends to be played in the Dorian mode

So what? Your generalization is still wrong.

On the two-page spread in Ryan's that has that tune, the keys/modes of the tunes are:

A dorian/minor hexatonic with pien F#s
E melodic minor (and the editor says "E MINOR" in case you might get it wrong)
G major
A minor
F lydian/major hexatonic
D major
G lydian/major hexatonic
G major
D major
G major
G dorian
E melodic minor
E minor
E minor&dorian both.

Not that unusual a mixture, then or now. (Some tunebooks of the time sorted tunes by mode. Ryan didn't, except insofar as he put tunes of the same rhythmic type together - hornpipes have less modal variety than other tunes).

BTW, Barbara has some decisions to make about Road to Lisdoonvarna, since every possible variation in mode gets played in current practice. The guy I learned it from said he didn't expect to stick to the same mode in two successive run-throughs. I don't think it was meant to be a session tune, it sounds better with that sort of soloistic spontaneity.