It is human nature to expect an object to be centered in its space. Just think of where your table is in your dining room, or where you have placed your bed, or how you arranged photos on a wall.
When a fireplace is not centered on the wall I assume it was for structural reasons, or perhaps the owners who built the house had a vision that we are not privy to.
One way I have been able to get around this issue is to place bookshelves or storage units on either side of the fireplace. An even better idea would be to build shelving units along the length of the wall and encorporate the fireplace into the shelving. The fireplace now has "purpose" in the space. It appears part of a unit. By placing identical bookshelves on either side of this fireplace it does not seem so out of place. The bookshelves will give your eye the balance it craves
Now here is a picture of a fireplace inset that is off center from the brick wall behind it. If you live in a house built in the early 70s you may have one of these in your home. This was before the days of plasma screen televisions - and before we liked to hang our tvs above the fireplace. So what do you do? Place book shelves on either side to give your eye some balance and then center the television on the wall. Quite simply... the object that is "off" is out-numbered by centered objects and the plasma tv now becomes the focal point.
KEY POINT: Unfortunately big, black plasma screen tvs usually become the focal point in any room.
This is my favorite image of what can be done with fireplaces the are not centered on the wall and also deals with the good ole' television set. In order to make the fireplace feel "naturally" off center they built a set of shelves or wall unit around it. The addition of the mantle that matches the wood on the shelves also makes it appear that this all had a purpose. AND... the television is hidden away in the cabinet so this beautiful stone wall and fireplace remains the focal point of the room. YEAH!