Firstly you can appeal the decicion to An Bord Pleanala for a small cost, this is worth taking a shot at. In the appeal to ABP, you should submit a cover letter specifying that your father-in-law owns the land. If you work locally or have ties to the land, this should also be included in the cover letter. They may overturn the decision to refuse. Often messy planning conditions are allowed to sit on the shelve.
If this fails, I would strongly advise you to write to one of your local councillor. Explain your situation... Having worked as a local authority planner for a few years, you'd be amazed at the bending of conditions when a councillor starts to throw their weight around. If there is some sort of unofficial agreement that is reached you may be able granted planning permission if you apply again.
Failing this, if you don't want to go down the legal route, you might try and hire a planning consultant. They often deal with such messy cases, they might be able to make headway by negotiating with the planners.