A Catholic priest was SUSPECTED of being involved. He was never arrested or questioned by the police. That leniency, rather than the silence of the Catholic Church, is the extent of the collusion or cover-up. The Catholic Church failed to make this public at the time? Given that there were no charges being made against the priest they could hardly have done otherwise. The people who covered up what evidence there was, to the extent that there was any evidence, and who failed to move against the priest were the British Government and the RUC. While the political fallout of putting a priest in the dock might have given HM Govt pause it was their decision and theirs alone.
And just to make matters clear I am a Protestant with a mother from NI and a grandfather and four grand-uncles who all served in the RIC. I prefer my justice in court and not by character assassination of the dead, who are (I hope, still) innocent until proven guilty.