Elopement

Mr Denis Lennon and Mary Mullin were actually eloping to the United States together, hence the shroud of secrecy and the reason why the two are listed as brother and sister.

The truth is Denis found a job as barman in the Mullin family's general store/pub in Clarinbridge, Co. Galway. He fell in love with Mary, however the upwardly mobile Mullin family forbade such a romance between a humble barman and their daughter.

The young couple, desperate and in love decided to elope to the States. Originally booked on the "Celtic" they were transferred to the "Titanic". They had been pursued to Queenstown by Mary's older brother who was supposedly armed with a gun (he had been tasked with retrieving their tearaway daughter). Nonetheless, the brother arrived by train too late and just missed the "Titanic's" tender. Hence the couple sailed off into destiny and disaster unhindered in their flight.

Sadly the two were lost, but the story truly echoes the fictional romance of Jack and Rose in the film "Titanic" - love across the class divide, pursual by a gun-wealding relative, eternal unity through death.....The only thing is - this really happened!

Source
Senan Molony, The Irish Aboard Titanic. Wolfhound Press. ISBN 0-86327-805-1