We're from an Irish background but in Australia. Dad left the RCC in his early twenties after what he witnessed growing up. My grandmother always hoped he would return. Here is why
For a couple of centuries Catholicism was banned in Ireland upon pain of death. People would meet in the woods secretly to have mass at rocks. Gaelic was banned, traditional dress, so was owning property, so was education, so was the right of Catholics to vote etc etc.
So for many Irish the assault upon the church was closely connected with the assault upon them and their cultural identity.
With respect, @Mimishimi, that is a classic emigrant mishmash of emotive identity politics and historical blurring. Most of the Penal Laws were repealed by the end of the eighteenth century. The Catholic Emancipation Act was in 1829.
After independence, the Catholic church and the Irish Free State and later the Republic of Ireland were so enmeshed as to make the Church the major stakeholder in the state -- not only did it dictate laws on divorce, contraception, abortion etc, it controlled most of the state's hospitals, schools and provided many of its 'social services', including, of course, running the mother and baby homes, the Magdalen laundries, the industrial schools which were part of the incarceration politics of Ireland for much of the 20thc.
Unless your grandmother time-travelled to Australia from the Penal Law era, she cannot really believe in Irish Catholicism as a badge of underdog colonial identity, when in fact it was essentially an arm of the state and firmly in control of the lives of its people for much of post-independence Ireland's history.