On October 12th, 1971, 19-year-old Una Lynskey stepped off a bus from Dublin to Meath for a short walk to her family's farm.

The walk should have taken her no more than 15 minutes, but she was never seen alive again.

Her body was later found fully clothed in the Dublin Mountains.

Accounts on the night talked of screams coming from fields, known locally as The Three Acres, as well as a mystery car in the area.

While several people reported seeing a Ford car in the area, there was some confusion as to whether it was a Ford Zephyr or Ford Zodiac.

But why was this distinction so important?

A Ford Zodiac Mark 4, 18-9-16. Image: Steven Bennett / Alamy

Martin Conmey who still lives on Porterstown Lane, was 20 years old in 1971.

Dick Donnelly and Marty Kerrigan were his best friends.

Dick was the eldest of the three. He worked on farms and building sites and was the only one of the three friends with a car - a battered 1964 Ford Zephyr.

The car was described as amber-gold following a dodgy paint job. It was also covered in rust and had a hole in the exhaust - but it was Dick's pride and joy.

Martin said the car was a great source of independence for them.

"I'd no car at all so I was depending on Dick for transport," he recalled.

"He had big speakers in the back window and these big 8-track tapes.

"He'd give it full blast and he'd deafen you... you'd hear it coming because the exhaust could be dragging on the road.

"There was holes and bits falling off them everywhere."

Martin joked that "you wouldn't even attempt" to take the car for an NCT.

Three quarter rear view of a Ford Zodiac car, 23-3-12. Image: Fotomaton / Alamy

The night Una went missing, Marty and Dick came across a parked car on Porterstown Lane.

They got a good look at the car and Marty thought it was a Ford Zephyr, the same as Dick's.

Dick, however, knew better. After spotting the twin headlights, he was sure it was a Zodiac.

It was very unusual to see a Zodiac parked on the lane like that.

He noted the number plate as best he could  - three letters followed by three numbers.

There were multiple sightings of the car throughout the night – and the question of whether it was a Zodiac or a Zephyr were to become significant to the investigation.

Its condition was also important, with some of the sightings describing a well-maintained and quiet-running car – quite different to Dick’s battered old Zephyr.

One of the witnesses - Una’s cousin Paraic Gaughan - said the car he saw was quiet, so much so that he barely noticed it until it was right behind him.

He also got a good look at the driver – describing him as middle-aged, well groomed and well-dressed in a suit and tie.

The front of a Ford Zephyr, 22-4-12. Image: Stuart Aylmer / Alamy

In the days afterwards another man - James Donnelly, a local farmer - came forward to say that he saw a large, dark-coloured Ford car speeding out of Porterstown Lane just after 7pm that evening.

He described it as being dark green or black in colour; an old type of car but well looked after.

He was also unsure whether it was a Zephyr or Zodiac, but he thought it had four headlights - suggesting it was a Zodiac.

The car was going so fast he had to slow down and vere to the right to avoid a crash.

When doing this he got a good look at the driver, describing him as middle-aged, with a reddish complexion, a stout build and balding hair, which was grey at the back and sides.

He also noticed a girl in the back of the car with another man, who appeared to be trying to kiss her.

In his words, she looked 'overpowered with fright'.

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