I have read that "fiddle" is an older word. Something like "fidel" or faithful.

"Violin" seems to have come from the viols in early music.

There is a difference. Observing great fiddlers, many do things that classical violinists would eschew. Holding the bow away from the frog, sticking the elbow out on the bowed arm, different bow holds, and the phrasing and vibrato (if used) are different. Mark O'Connor used to put his thumb under the frog area rather than inside..a technique learned from Benny Thomasson. (Texas fiddle).

Ti Jean Carignan was a phenomenal fiddler from French Canada. Many classical violinists would see him and be amazed that he could do what he did (they couldn't) "all wrong".

The analogy above: blues guitar players and classical guitar players. Oranges and apples.

The cross-over seems difficult. Stylistic approach is different and one not conducive to success in the other. The European bow holds almost preclude getting a "lilt" to Irish fiddling or the drive of US rural styles.

Watching Mairead Nesbitt with the Celtic Women, it seems that she holds the bow up from the frog, sticks her bowing arm out at the elbow and gets a beautiful sound that lilts.
She may have had classical training but she has a fiddler's approach.

Frank Hamilton