Place your pivot point in the center of the location you want to mirror around.

It can help to freeze your object's transformations, especially the Scale Channels.

Duplicate and put -1 (or, if you do not freeze transformations, make whatever amount is currently present negative) in the scale channel that is paralell with the direction in which you want to mirror.

It can be good to Freeze your newly mirrored object's transformations as well.