I was wondering what kind of embelishments work best on repeated notes. I’m currently learning the Swallowtail Jig and the Kesh Jig, and both of those songs contain lots of repeated notes. What could I do to them other than just tonguing them?
Hi there 
Cuts or strikes can be used for separating the same note. A cut is a quick flick of the finger; so if you had F F, you would play the first F and keeping the stream of air bring your right hand index finger up and down quickly; just enough to distinguish two notes. A strike does the same but going a note down instead of up. It’s a little wacky to try and explain hahah but if you go to a music store or look online, there should be some good books to explain different techniques. One I would suggest is a book a friend had lent to me: The Clarke Tin Whistle by Bill Ochs. Otherwise, mess around a bit and have fun experimenting with the songs!
Hey Pie dude - go here
http://www.rogermillington.com/siamsa/brosteve/
There’s all you need to get started - keep that tongueing down to the minimum 
As sugested, there is the Bill Ochs book, also the Grey Larson books and a plethora of others - many very good - search this site for “embellisments” and “Tutorial books”.
Another thing to do is watch highland pipers - these guys are truly amazing at stutterizing notes! And they have romantic Gaelic names for every stutter. Whistlers can be content with cuts, strikes, rolls and a cran or 2 I’m sure there’s more for the adventurous.
(edited to say: stutterizarigamarollingarizinatinmentalmentingating)
I think rolls sound wonderful for the Kesh jig, particularly those repeated G and A notes in the beginning of the A part.
I agree with the cat here - have a listen to the way Wanderer does it:
http://www.tinwhistler.com/music/mp3/kesh_jig.mp3
In the second part of the first bit Try to cut-into the 2nd octave E and D for good rhythm using the top hand 3’rd finger cut. It also helps the breath rhythm to do that. Also, cutting into D is a real challenge on most whistles - this will need lots of practice to stop the whistle from wadering into the many harmonics the D cut can cause - specially in octave 2 - you will need to get this under control before you can cran on that note.
Don’t forget the variation in the 2nd time through both the A and b parts - otherwise it sounds a bit boring.
In the B part, the high B can be cut into or slid from the A - this makes for good variation but the cut should be learned first to develop the B - G drop-down, your fingers will need to learn this one and it gets muddled when you use the a - b slide going into the high B - G F#G phrase. Keep in mind that the slides are very pretty, but sometimes do not have sufficient dominance when you want to emphasize for variation - in these cases a cut works better, so save the cut for the 3rd time through.