Depends. Block, static mesh or animated mesh?
I would assume a fireplace would be a block, in which case it is a touch complicated. I have written a guide here:
https://github.com/MovingBlocks/Terasology/wiki/Block-Shapes-In-Blender . You can also look at the existing blocks for examples. A fireplace would probably just have a center mesh and a bottom mesh components - the different mesh parts are used when the blocks are stitched together to form a chunk (the bottom will be skipped if it is rendered invisible by being adjacent to a surface below it.
Then you save the mesh as a terasology shape (as made available by the Terasology addon) and place it in a shape folder in a mod.
I realise that is quite a brisk answer, feel free to ask for clarification on anything.