You should be aware of the limitations of your instrument. Like a tin whistle usually has 2 octaves range and is diatonic, so has only 7 of the 12 notes that a chromatic scale would have per octave.
So pieces that you want to play should also stay within 1 or 2 octaves and shouldn't leave the key. If that is the case you can transpose them from one key to another. Which essentially just means that you move every note up or down a few half or whole steps, keeping the intervals between the notes (aka the melody) the same but changing the start. So it will sound a little higher or lower, which only really becomes relevant if your audience has perfect pitch or if you're playing with other instruments or have people sing to it otherwise people usually focus on the melody which stays the same.
If that is not the case then you need to be a little resourceful. If it has more than 2 octaves you might be able to get away with folding it back to 2 octaves so playing the note but an octave higher or lower than indicated. That will sound different as you change the direction of pitch (high-low to low-high or vice versa) but usually still somewhat pleasant as you keep the interval between notes. Need to see if that works for you and it's often just a few notes for which that is necessary.
If you have key changes within the music you might track whether they are random or whether you can identify sections and then transpose each of the sections to your favorite key and then figure out how to transition between the section, like idk a pause, a common note between the keys, just the root note of the new key to establish that or something like that.
You can also try to play just idk every second note just the ones that you can play and see whether that's already similar to your expectation or not, or doodle along to a recording trying to find the essential parts of the piece that make you go "ah that's ...".
It's not impossible to play sheet music for a different instrument or genre, but it usually involves some tinkering.