|Does the phone in the barn terminate in the house? Did you have to put a filter on that phone line in order for the phone to work?|
If the answer to the two above questions, it may be possible to purchase another modem from your provider and connect it to that phone line in the barn.
I'm not sure this would work. It might be that it will but your ISP would consider it a second, separate DSL connection which you would have to pay the usual monthly fee for. If that is the case, take a look at some wireless antenna's as Richard59 said. In the long run, if you had to pay for a second DSL connection, the antenna's would likely be more cost effective.
I think it would be worth calling your ISP and asking them about the possibility as the phone call itself isn't likely to cost anything and if it could be done without it being considered a second, separate DSL connection, you could save yourself some $$$.