Irish migrants to Britain were often young and single and lots of them weren't as Catholic as all that. In many places there were no Catholic services within walking distance. Many people who settled locall y with an English partner ceased to be Catholic in practice. Priests preached against mixed marriages but were also aware that you gained families as well as losing them.
Irish Catholics were doubly a minority and minority numbers are always crumbling as individuals integrate with the majority. Many people who were probably Catholic had Anglican marriages and burials and (to a lesser extent) baptisms.