I don't think there's enough information to make a decision about Brady's actions in 1975. It may be one of those "looks worse than it was" situations, or it could be an intentional coverup. I'll wait and see.

The situation Brady found himself in in 1975 is known as was his function and role in it.

Cardinal Brady himself has declared that armed with hindsight he should have acted differently. The victims of Brendan Smyth obviously disagree. Specialists in canon law like msgr Maurice Dooley on the other hand are quick to declare that the Cardinal did nothing wrong at all at all.

Whether or not one feels Seán Brady should have acted in 1975 hinges on how you weigh the obligation of those involved to report the abuse to the Gardaí as opposed to swearing two abused children to secrecy.
You may or may not choose to consider the argument that at that time people didn't know how to deal with child abuse (see an interesting letter to the editor in Saturday's Irish Times).