That looks good, and it looks like it's part of the original wall.
If you compare the lath that you've revealed with a piece of lath from another part of the building (the other side of the ceiling, from the loft, would be a good place to start), you should be able to see if it's the same or not.
Common timbers used for riven lath were Oak and Sweet Chestnut. Scots Pine has also been used for lath, but was normally sawn. I reckon you should keep the lath and plaster and repair the part you've taken off, it looks ok to me. The wall should dry out with the new pointing, repairs to the chimney and the dry heat from the wood burner.
If you're thinking of lime wash, you can get all sorts of colour from here........
http://www.cornelissen.comPure lime putty mixed 50/50 with water works well, but if you want to paint timber or any other material with it, you'll need casein to mix in. I would just paint the walls, run moulding and ceiling with lime wash with a colour in it, as the casein mix is not as moisture permeable.