Don't click the link thinking you'll see a scholarly article that you can pick apart and argue about. These are differences that almost anyone can spot, but if you play, you'll nod knowingly while chuckling. A sample:
The Bluegrass fiddler paid $10,000 for his fiddle at the Violin Shop in Nashville. The Celtic fiddler inherited his fiddle from his mothers 2nd cousin in County Clare. The Old Time fiddler got theirs for $15 at a yard sale. Celtic and Bluegrass fiddles are tuned GDAE. An Old Time fiddle can be in a hundred different tunings. Old Time fiddlers seldom use more than two fingers of their left hand, and use tunings that maximize the number of open strings played. Celtic and Bluegrass fiddlers study 7th position fingering patterns with Isaac Stern, and take pride in never playing an open string. An Old Time fiddle player can make dogs howl & incapacitate people suffering from sciatic nerve damage. An Old Time fiddle player only uses 1/8 of his bow. The rest is just there for show.
Personalities and Stage Presence
Bluegrass band members wear uniforms, such as blue polyester suits with gray Stetson hats. Old Time bands wear jeans, sandals, work shirts and caps from seed companies. Celtic bands wear tour tee-shirts with plaid touring caps. All this head wear covers bald spots. Women in Bluegrass bands have big hair and Kevlar undergarments. Women in Old Time bands jiggle nicely under their overalls. There are no Women in Celtic bands, only Lassies with long skirts and lacy, high collars and Wenches in apple-dumplings-on-a-shelf bodices and leather mini-skirts. A Bluegrass band tells terrible jokes while tuning. An Old Time band tells terrible jokes without bothering to tune. Bluegrass band members never smile. Old Time band members will smile if you give them a drink. A Celtic band is too busy drinking to smile, tune or tell jokes. Celtic musicians eat fish and chips, Bluegrass musicians eat barbecue ribs, and Old Time musicians eat tofu and miso soup. Bluegrass musicians have mild high frequency hearing loss from standing near the banjo player. Old Time musicians have moderate high frequency hearing loss from sitting near the fiddler. Celtic musicians have advanced hearing loss from playing in small pubs with all those fiddles, banjos, tin whistles and bodhrans.