What does this mean? Is it a sarcastic phrase?
This is very interesting. I think of it nowadays as mildly sarcastic, but I guess it can also be a statement of support and encouragement (in the form "More power to you!").
For example, this was found at Google Books. Judging by the oldest results there, could the phrase have an Irish origin?
The phrase sounds like a brush-off, well, I really don't believe you can do it, but whatever. No matter how you say it, it does not sound supportive, but rather the opposite. I can't find a definition anywhere.
Hmm, I've always used this in a much more positive sense:
If someone thinks that something about themselves is not so hot and I disagree, I might say "Hey, more power to ya!" to tell them so.
On the other hand, I can see how, depending on how it's used, and particularly how it's SAID, it could be spun in a number of ways.
I cannot recall hearing this phrase used in a sarcastic manner, though I can imagine how it might be interpreted as such. I have always heard the phrase as a reply to a person who has just described how he or she has decided to act in a certain situation. Example: "The next time he teases me about my age is when I'll ask how his toupee is holding up."