This is one of the hits from Dire Strait's album Communiqué, released in 1979.

Some people get a cheap laugh breaking up the speed limit
Scaring the pedestrians for a minute
Crossing up progress driving on the grass
Leaving just enough room to pass
Sunday driver never took a test
Oh yeah, once upon a time in the west

Yes it's no use saying that you don't know nothing
It's still gonna get you if you don't do something
Sitting on a fence that's a dangerous course
Oh, you could even catch a bullet from the peace-keeping force
Even the hero gets a bullet in the chest
Oh yeah, once upon a time in the west

Mother Mary your children are slaughtered
Some of you mothers ought to lock up your daughters
Who's protecting the innocents
Heap big trouble in the land of plenty
Tell me how we're gonna do what's best
You guess once upon a time in the west

Oh yeah, once upon a time in the west
Oh yeah, once upon a time in the west
Oh yeah, once upon a time in the west

Once upon a time
Once upon a time in the west

Once upon a time in the west
Once upon a time in the west
Once upon a time in the west

Once upon a time in the west
Once upon a time in the west
Once upon a time in the west
Once upon a time in the west
Once upon a time in...

This song is about the dangers of living in London's West-End area. Mark Knopfler compares life in the West-End with life in the American far west. You have to be careful with wild drivers; life is cheap, even if you think you are a hero that doesn't make you invincible, etc.

The reference to Mother Mary's children being slaughtered may have different interpretations. She could be the Virgin Mary, so her children are everybody, or she could be just a woman called Mary, used as an example of any mother, or it could even be the same image used by John Lennon and be a metaphor for drugs.

Anyway, the message of the song is: London's West-End is just as wild as the American wild west.