That is almost like asking for THE recipe for potato salad - LOL. Every family, every area will be different. You start with potatoes, cabbage, butter and cream - then go for it. Many add a bit of cooked bacon, some spring onions and use milk instead of cream. Some add kale or a bit of spinach. Years ago I started to always add rutabaga or cauliflower to my regular mashed potatoes and so I naturally add them to my colcannon. Also a bit of nutmeg to any mashed potato item helps enhance. Again, it is what your family likes to eat. The most important part of this dish is that it is eaten with the family gathered around the table and enjoying each other!