Hi! I am a beginner, trying to learn to play the tin whistle!
I just recently got quite confused.
When you search for tin whistle songs on the internet, the songs are made in a dot-system, or something like that.
Now i know, that ‘d’ is the most normal key of a whistle, but can the dot-system songs only be played on ‘d’ whistle, or can they also be played for example on a ‘Bb’ whistle? Do i need to transpose? 
Sorry if this is a stupid question!
Before we can answer your question you may need to explain further what you mean with ’ a dot system’.
I take it you mean the tablature that shows you which fingers to leave on the whistle (or which to take off, depending how you look at it). In that case you can play as written on any whistle, you’ll just be playing in a different key.
Yes, that was what i meant! Thanks 
Transpose? When I began learning to play the whistle a few years ago (look over to left that’s me, still newbie-ish) the word transpose didn’t even exist in my vocabulary. Say what? And, I still don’t really fully understand it, transpose, keys, tabs, staff, ABCs, yikes.
Have fun with your whistling, and welcome to the whistle forum.
Can’t add anything to what Gumby says, but in the spirit of “helping the newbie”… They aren’t “songs” - they’re “tunes” (unless they really ARE sung). Just trying to remove any future embarrassment!
Good luck in your journey!
Pat