Here's an example of a sincere use. If somebody mentioned it to me I might say it aloud. If a Facebook friend posted something about it, I might post this in reply. News: "Taylor Swift has donated $1 million to help Louisiana flooding victims." Me: "Good for her!"

I say "Good for you" both genuinely and sarcastically. If someone says "I passed the test" I might say "awesome" or "good for you" But if someone says "I have much more money than you!" In a boasting manner I would say "good for you" sarcastically.

Maybe it's just me but I can't say that phrase without feeling like I'm being patronising.

It definitely can be sincere. I use it sincerely myself. However, I mostly use it in the third person, to mean strong and heartfelt appreciation. "Did you hear that our local bank donated $50,000 to the high school scholarship fund? "No, I hadn't. Hey, good for them!" In the second person, it is usually a little too strong to use with an adult; you would be more likely to use it with a child. "I got 100 on my science test!" "Really? Good for you!" In fact, the reason why it is good as irony is because it is often sincerely. To tell which it is, you have to sense the context and the tone of voice. "Politician XYZ said thus-and-such." "[Snort] Well, good-good-goodie for HIM." <---Ironic.

The phrase can be said approvingly, but most of the time I hear it when adults are talking to children (after they've just accomplished something). The phrase is used sarcastically usually when someone hears another person boast about something that they achieved. Example: Bob- "Hey, I know you no one has asked you yet, but five girls just asked me out to prom!" Jake- (sarcastically) "Good for you!" Most kids/teenagers will say it sarcastically, though honestly people mean it both ways all the time. Just be careful how you say it, because if you do actually mean it approvingly, and you come off as being sarcastic you can hurt someone's feelings. Hopefully that makes sense.