Why do Irish & British sausages have soft casings?
There are many potential variables here, particularly if the sausages are made with natural casings.  More likely it has to do with the actual casing source.  Casings from beef are different consistency from casings from hogs and from lambs.  I have found that hog casings are the softest that I have used, but I am in the US, not in Britain or Ireland.  I have also found that casings from different suppliers are different.  The ones I can buy locally in small quantities tend to be much tougher than the ones that I purchase online from a commercial provider.

I am guessing you have a situation where the sausage makers in Britain and Ireland have access to very high quality casings that are softer.

Some of this may be due to the diet of or the breed of animals that provide the casings.