This is where I'm trying to get a wee bit more technical in the answer.
This is where you open up a can of worms. Firstly it isn't just airfield height temperature and runway lenght which governs the max take off weight, but also runway slope and wind. If there is a stronger headwind then the weight will increase, and vice versa. The prevailing wind will determine which runway is in use and this in turn may have a lower max take off weight because of terrain problems on climb out. If there is high ground off the end of the runway then the aircraft has to be able to clear it on one engine if the other fails on take off. Add these factors together and you can calculate your max weight, but even this won't answer your question for several reasons. Fistly if you are operating a charter aircraft then you probably have 5 seats for every 4 on a scheduled aircraft. If you are full then that may mean an extra 30-40 seats which in round numbers may add four tons to your take off weight. Passsengers, depending on the regulatory authority are also taken to be a standard weight, one for males and one for females, so if it is a rugby charter then a different weight would be used than on a nuns charter to Lourdes. Someone may have a performance manual to hand to give you a max weight for the runway but that may still not answer your question fully. Catania to Dublin is quite a long way as shorthaul flights go, and the reason given does not seem improbable to me. Having said that I am no expert on the performance of the MD83