enviromentaly friendly deep pit outhouses

We have a cottage on an island off the coast of Ireland. The living at the cottage is very old fashioned with gaspowered lights and cooker and no toilet. We have been using temporary pits which which we dig everytime we go to visit and when we leave we backfill the pits and burn the toilet paper.. This is a task that we don't want to have to do everytime and I am researching how to build a deep pit toilet to the rear of the property and would love advice from people who have buit or plan to build an enviromentally friendly outhouse - everything from construction techniques of the pit, any enviromental concerns which arise, the longevity of the pit on a site which is really only used during the summer for maybe 3 to 4 days at a time....Can anyone advise