Aideen Hayden (Labour) | Oireachtas source

I will go back to basics. The word "Schengen" is tossed around a lot, and although I thought I understood it, having listened to Professor Maher, I realise I do not. Professor Maher said the Schengen accord removes internal borders and establishes a common external frontier and, thus, common visa, asylum and immigration policies. The reason the UK would not consider entering Schengen was the immigration aspect of it. Given that I am not an expert in European law, protocol 21 is new to me. From both presentations, it appears that it covers many of the areas that Schengen covers. Both Ireland and the UK have a right to opt into it and the UK has opted into a number of the areas. Immigration remains the area that seems to cause the UK the most difficulty. Am I correct?