We'd like to remove the top two rows of brick to lower our mantel (we want the TV above, but currently it's a bit too high for our taste). however, when I removed the current mantel I discovered that behind the brick facade appears to be poured concrete. I'm not sure how far down the concrete goes...

Should I still be trying to take the top two rows out or is this a lost cause??

you might still be able to remove the two rows of brick, and use a concrete saw and chisel - and cut/smooth down the concrete to the required level.

HOWEVER - before you do this - you would need to examine the outside structure carefully. And probably have a chimney person come on out, and give you some advice, or what he thought options would be, and how much possibly they wouold charge.

Reason being - a wood burning fireplace needs a chimney. A true birck chimney WEIGHS a lot! Concrete has been used as a foundation for a brick chimney. So you need to know what the concrete is doing, purpose of, etc. before you go cutting into it. But I have a lot of concrete tools, and concrete cutting tools do exist, so it could be cut... just need to know more stuff before you go messing with it.

Might even ask a structural engineer to come on out, and take a look. Depends on how bad you want to watch Frasier with the TV at the desired height :)