Evil twin killed mum, wimpy twin went to clean up mess and shot dad (opportunity arose). In court, wimpy twin tried to convince everyone he did shoot his dad, evil twin said it was all him.
Jury found evil twin guilty and wimpy twin not guilty.
Much shouting ensued.
Wimpy twin walked out court and went outside and raised an evil grin. So I think they had swapped places.
I may be wrong though!
As one of the twins was being led down into the cells at the near the end he was seen to use his inhaler which implies that they did indeed switch and that the guilty twin walked out of court, hence the smirk as he walked away. Wonder how long it would have taken them to realise that the wrong twin was locked up? If the prison officers displayed the same abilities as Roisin Connor then quite a while I should imagine.