The problem would get worse, at least you can see it now. Are you sure it is condensation? if so extra heat and ventilation would help. We had a damp farmhouse but we stripped all back to stone extended around it and externally insulated with new lime plaster on the inside. The previous owners had a damp problem stuck a layer of terracotta brick slips over the damp which was coming from outside. It hid the damp but you could smell it even though you could not see it. I would not do any work until you have done a full assessment of the problem. What is on the external of the wall? Our situation was made worse by fact the external and internal wall had been cement rendered so wall could not breathe. Some thoughts. Did a previous owner convert the barn are there any drawings? you could consult. Do building control have a record of the work.?

A good thickness of insulating lime plaster internally (or can it be external too, or instead?)

Posted By: sellyI've got a nice old lime plastered converted barn. But its cold. And then the condensation gathers in a lot of places and then the lime goes black. To be honest i hate living here now.
How much would I ruin the fabric of the building if I dot'n dabbed insulated plasterboard onto the internal walls?

EPS Insulated plasterboard's sufficiently vapor open and works well when installed properly. Seal board perimeters and around wall sockets with expanding foam to restrict air movement. Ventilate the house well to dry the walls and treat the mold. Dig into wall areas with condensation to rule out rising damp or a gutter leak, condensation's usually on the surface.