The show Riverdancing doesn’t involve Irish dancing in a river, but there are jokes. “Did you hear about the 25 Irish people that drowned? They were riverdancing” was posted on Twitter on September 13, 2009.
   
“I saw an Irish dancing show today called Streamdance. It’s not quite as good as Riverdance, but then it is only a tributary act” is another Riverdance joke.
 
 
Wikipedia: Riverdance 
Riverdance is a theatrical show consisting mainly of traditional Irish music and dance. With a score composed by Bill Whelan, it originated as an interval performance act during the 1994 Eurovision Song Contest, featuring Irish dancing champions Jean Butler and Michael Flatley. Shortly afterwards, husband and wife production team John McColgan and Moya Doherty expanded it into a stage show, which opened in Dublin on 9 February 1995. Since then, the show has visited over 450 venues worldwide and been seen by over 25 million people, making it one of the most successful dance productions in the world.
     
Twitter
Judith Ann
@prissy121
Did you hear about the 25 Irish people that drowned?
They were riverdancing.
8:06 PM - 13 Sep 2009
 
Twitter
Jean Jepson
@bluejean104
Replying to @Swannyg66
@Swannyg66 30 irish people found drowned, police believe they were river dancing.
11:15 AM - 24 Sep 2009
       
Twitter
Ian Walters
@Hugh_G_Rection
tried river dancing last night, almost drowned!
3:58 AM - 15 Oct 2009
 
Twitter
Gavin Edwards
@GavinEdwards_
I tried River Dancing once…. Nearly drowned , Bad Tequila..
6:34 PM - 29 Jan 2010
   
Twitter
Thomas White
@JusticeRulesTW
I tried River Dancing once, I almost drowned.
1:41 PM - 11 Jan 2011
     
Twitter
Janet Cardin
@janetlcardin
@Alicebonniesmom Q: Did you hear about the 25 Irish people that drowned? A: They were river dancing.
Sent from 20000 Jokes on Android
11:57 AM - 6 Aug 2011
 
Twitter
Jokes Forest
@JokesForest
Q: Did you hear about the 25 Irish people that drowned?  A: They were river dancing.”
4:24 AM - 25 Jan 2012
       
5 October 2013, The Daily Mirror (London, UK), “Jacklin likely to miss the ballroom” by Derek McGovern, pg. 8:
My mate was a big fan of river dancing. That was until he drowned.
 
Google Books
Sensible Nonsense.. Frank’s Comic Pack:
Humour Pack Series
By Frank Paul
BookBaby
2014
Pg. ?:
Q: Did you hear about the 25 Irish people that drowned? A: They were riverdancing
 
Twitter
lorraine Jo King
@LahLahBean
#sidesplittingsunday
Did you hear about Paddy drowning.
He was river dancing.
4:21 AM - 24 Jun 2018