Most of the songs that I play on my whistle don’t have any extra accedentals but the few that have the odd sharp or flat I was wondering how you could play them. I’ve heard that you cover half the whole of that note, but that doesn’t work to well for me. Does anyone have any suggestions?
I suppose you mean sharps and flats before single notes. You can play them covering the half hole but some can be played with cross fingering. You can find fingering charts for that and then you will have to try because the result may depend of the whistle or tone.
To improve the “half-hole-covering” tecnique you could try to play the scale slowly in “half tones” up and down. Instead of trying to cover a half hole you can just lift the finger away a bit of the hole respectively cover by approaching the hole slowly. It is not easy and needs practice. I have also a lot of problems with that 
I whish you succes.
Silvano
You have to keep practicing. If you’re trying to play G#, for example,
start by playing G and slowly rolling your fingertip that covers the G
hole off of the whistle until the note sounds like a good G#. Keep
doing this until your finger “remembers” where it has to be to make
a good G#, and you will eventually be able to do it quickly. I agree
it’s not easy, but whistle is not a chromatic instrument, so you really
have little choice but to work around that with practice.
As Silvano said, there are a few crossfingerings, but they are limited
to a few notes, and vary widely by whistle. The only one I rely on is
the Cnatural crossfingering, everything else I have to half-hole.
i also recommend playing along with another musician on another instrument or along with a cd. that might give you a push to hit the accidental better. the other music helps get me to where i need to be.
Rolling your finger off works in some situations but less well in others…with time and a lot of practice you can also train your fingers to hit a half-hole directly. I know a great whistle player in Vermont who worked on this for years and I’m always amazed listening to him play hornpipes with G-sharps and Eb’s on a D whistle, he gets those notes spot-on in tune every time, no sliding, just a direct crisp hit on the half-hole. He does F-naturals that way too, although he can slide into them for effect as well.
If you think about it, fiddlers have to deal with much the same issue of training their fingers where to land. There are no frets on a fiddle!
Perhaps I’m wrong but I think this is easier to learn when you play longer on the same whistle. I. e. the gemetry of whistles (hole distances and bore diameter) in one key varies quite a lot from maker to maker which is a notable difference to fiddles. Changing the key makes it even more difficult. All one can do about is practice, practice, practice.
I wonder why we are here typewriting instead of doing that 
Silvano
This is a sort of odd and fun website which has charts of many alternative fingerings. Some might work on your whistle for accidentals. Halfhole fingerings are included so you could try all the possibilities to see which sounds most in tune on your whistle. Your ear would have to be the judge.
http://www.fullbodyburn.com/index.htm
Which song(s)?
I was reading down the thread, and I was wondering when somebody was going to mention the fingering charts.
I found difficulty doing half-holeing myself, but I found that if I just straightened out my finger, I get pretty close to the incidental consistently. Probably a symptom of my incorrect grip. 
True. I mentioned the finger-roll only as a way to find the position that
you need for the correct tone. Once you get used to where this is, you
can hit the half-hole accurately with practice. If a tune calls for it, you
can slide up into the accidental, but as brad says, this is not always
possible, for instance, in Coleraine
Thanks! I went to the website and some of the alternate fingerings work, for the rest I will continue practise the rolling. 
I desperately wish I hadn’t used the word “rolling”, since that’s already
the name of an ornament. I should have said “sliding the note sharper”
or somesuch. I apologize to all those who are now confused.
To half-hole notes becomes very easy if you practice scales and or songs that require them. Don’t give up.