Hi Alaniho,
There is a system new to Ireland for insulating between the leafs in a cavity, it has a long history in the US and Canada, the problem with cavity wall insulation is it fills the cavity, which is there for a reason, to control vapour drive, you can insulate with any number of foam board products on the market, but these to have limitations, if not installed correctly they can fall down through the cavity and cause cold bridging.
You can spray the inner leaf with a closed cell foam to a dept of 50mm, it is the same material as foam boards used for this purpose, but it forms an air-tight barrier, and is monolithic which means it's one piece, so it will never move or fall down in to the cavity, it can eliminate cold bridging and draughts, closed cell is also a vapour-retarder.
But remember it must be CLOSED CELL foam, I hope this helps