Not exactly fiddle tunes, but a couple previously discussed here were:

The Man Who Turns The Damn Thing Off And On

I Just Don't Look Good Naked Anymore

and there's:

Nobody Loves a Fairy When She's Forty.

There should be threads on these three(?).

In a different genre I think I recall a thread that ran for a very long time before we found:

Whose Izzy Is He (Is He Yours or Is He Mine?)

A "Session Book" circulated some years back by the "Irish Tent" at the annual Winfield Festival (and elsewhere) included:

Britches Full of Stitches
Fasten the Leg in Her (mentioned above?)
The Flogging Reel(?)
The Hag At The Churn
I Buried My Wife & Danced On Top Of Her
Munster Buttermilk has the alternate title Behind the Haystack which sounds mildly salacious?
The Nine Points of Roguery
The Price Of My Pig
The Scent of the Bog (The Smell of the Bog)
Saint Anne's Reel is well known but St. Ruth's Bush possibly not?
Toss The Feathers(?) (multiple versions)
The Wet Pussy (An Phis Fhliuch)

Some in this latter collection have alternate titles that might sound funnier if I was in "full Irish mode" but I wouldn't be able to tell if the silliness was me or the tune.

John