The event is not the tension
If the reader already knows the ship sinks, a chapter that only waits for the iceberg is a countdown, not a story. Countdowns are thin. The tension is the dinner they are still enjoying, the marriage they are still postponing, the letter they are still not sending, while the reader watches them spend time they do not know is short.
History supplies the clock. You still have to supply a human use of the minutes. Ask what this person would do differently if they knew, and then do not let them know. The gap between their plan and the reader’s dread is the scene.
Do not wink
“Little did she know” is the wink in costume. So is a narrator who nudges — the unsinkable ship, the last peaceful summer, the arch remark about a war that will be over by Christmas. The reader does not need to be congratulated for knowing the ending. Congratulation turns dread into smugness, and smugness is the enemy of pity.
Stay in the character’s weather. Let them call the ship steady because everyone they trust calls it steady. Let them plan a harvest that will not be brought in. The irony is stronger when the sentence is innocent and only the reader hears the crack.
How far ahead the reader should be
A few steps, not the whole map. If every page is coloured by a doom the reader can already date, the middle of the book becomes waiting. Give the reader one fact the viewpoint lacks — this letter is a lie, this cough is not a cold, this road leads to the army — and let the scene turn on that ignorance.
You can also reverse the usual gift. Sometimes the character knows the room and the reader does not: a gesture, a dialect, a silence that means danger only to them. Irony is not only the reader looking down. It is any unequal knowledge that makes the next choice hurt.
When the history is not famous
Most novels are not about the Titanic. The reader does not arrive knowing the parish, the battle or the bankruptcy. You can still build irony without a public clock. Let the reader see a self-deception the character cannot: he thinks he is being prudent; we have just watched him ruin the person he means to protect. Private irony needs no Wikipedia.
If you do use a famous ending, decide what the book is about besides it. The sinking, the execution, the crash of the market should revalue a choice we have already watched someone make. If the history is the only thing that happens, you have written toward a fact, not toward a person.
Try this
Mark the one fact the reader has that the viewpoint does not. Rewrite the next scene so that a decision depends on not knowing it — and delete any sentence that winks.