Most of the "Irish Traditional" practicioners, even the wannabees, are fairly consistent in using "song" to mean something you sing, and "tune" to mean something played on an instrument. This distinction is not quite as apparent in other traditions, but probably is followed to some extent.

Early Irish, that I've seen, seems to have both kinds of "song" traditions. Large parts of the repertoire are considered, at least by purists, songs to be sung unaccompanied. Other kinds (e.g. Carolan's odes) were quite obviously composed by persons who accompanied themselves, or were accompanied.

"... leaving spaces for fills" is a bit ambiguous, since it could refer to an "instrumental break" or could refer just to "ornamentations" within a tune. Of course neither is applicable to unacompanied singing.

Where an instrument is appropriate, one would suspect that an instrumental break - "playing through" a verse without the vocal - quite probably would have been sometimes part of the performance, if it was a nice tune; but I'll defer to more knowledgeable comment on that.

Any instrumental part is likely to have included "ornaments" such as trills, grace notes, etc., since these are essential to being able to play some instruments smoothly. The particular embellishments vary with the instrument, and likely would vary depending on the skill and preferences of the performer, so saying that a specific trill, turn, or gliss is "part of the tune" is akin to saying that tune should only be played on the instrument for which that bit of fancy works.(?)

A reasonable exposition on ornamentation appears in the preface to Klassen's republication of O'Neill's Music of Ireland (Oak Publications). O'Neill, the collector, was reportedly a good flute player, but didn't read or write music. His collaborator, Sgt. O'Neill (no relation) who actually did the notations, was basically a "non-player" but could do the notation. The original publications of the collection (ca. 1903 - 1920) were criticised by some for including "excessive ornamentation." In part that may have been because O'Neill put in "flute ornaments" when many of the tunes were generally "fiddle tunes," so they were the wrong embellishments for players of other instruments.

Klassen's "edited" version was criticized both for removing a lot of ornamentation and for adding ornaments "more appropriate to the fiddle." Klassen provides a reasonable explanation of what he'd done, but apparently a lot of people never read the preface...

Ornamenting to suit the individual player presumedly has always been a part of traditional instrumental music, since you can't play the tunes on at least some traditional instruments without the ornaments appropriate to the instrument.

John