The last event states Ireland "returns to rugby glory" suggesting it had won the grand slam before, but also there was a decent amount of time since the last win (return suggests a hiatus). This would exclude the original 1948 win.
In 1948 it was the 5 Nations, not the 6 Nations (I assume that the question hasn't been rewritten since you saw it)
In 1948 it was the 5 Nations, not the 6 Nations (I assume that the question hasn't been rewritten since you saw it)