Open the sliding door halfway and lift it up, about 1cm of travel, swing bottom out and let back down. The top will come free and thats it, door free.
Put the door on its side and clean out around the rollers. If they are worn you should replace them. A&L Castors in dublin would be a good place to start looking for parts.
A good cleaning and some copper grease should do the job to release and encourage the doors to slide again. Don't forget to clean the runner track removing any grit and dirt that has built up there.
Simply reverse the lift out to get the door back in.