She looked as fit as a fiddle and was brilliant at her job.
But her face didn't fit any more, so she had to go.
Or, to be more precise, she got the boot because of the lines on her face.
O'Reilly had been a manager at the Nat West, a high school geography teacher or a shelf-stacker at Asda, no boss would have even considered sacking her just because she'd got a few crow's feet and there was a younger, fresher-faced applicant waiting in the wings.
But Miriam wasn't in office admin, or retail. Or any of the ordinary, steady careers that millions of women her age are still happily and productively engaging in, their wisdom and years of experience still gaining the full respect of their bosses and peers.
She worked in television. That glamorous world in which people can swiftly become a household name, an instantly recognisable face, and earn mega-bucks for doing what most people would give their eye teeth for.
She was the presenter of BBC's Countryfile, admittedly a rather less glamorous show than most. But still, she was on telly, being famous, and being paid a lot of money in return for giving a relatively small amount of her time to climb mountains, ramble some of the UK's most stunning countryside, learn to waterski. And do some interviews.
But then, in 2008, the Beeb decided to make its gentle, Sunday morning show a bit snappier, a bit more exciting – and give it a "prime time" slot. So changes had to be made. Out went Miriam, now 53, along with other female reporters who had turned 40 - Michaela Strachan, Juliet Morris and Charlotte Smith.
And in came smiley and less wrinkled faces in a bid to pull in younger viewers. (Did it work? I'd love to know. My view is no-one under 50 who doesn't live or work in the country is remotely interested in the show).
Only John Craven, old and craggy as the hills, remained as the link to the past – and the token oldie.
Outraged Miriam decided to do what former newsreader Selena Scott and Strictly Come Dancing's Arlene Phillips didn't; she took the BBC to a tribunal, citing ageism and sexism.
And by gumboots, the country lass (well, she is to me) won – on the ageism bit, anyway. Her victory is proof that TV does have a downer on age. That such injustice has been going on for years.
It is totally wrong that any woman who is hugely competent at her job should lose it just because she's looking older.
But hang on.. Don't the women who bust a gut to get themselves on to the telly know that ageism, and job insecurity, and getting your freelance contact axed on the whim of the boss, is part of the package from the start?
You're not telling me they don't understand precisely that what they are getting into is a fickle, image-led world where looks matter way more than they should.
The BBC said after losing its case that the tribunal didn't understand how TV works, how fashion-conscious TV has to be.
It sounded pathetic. But heck it is the truth.
Because, and here's the nub, the looks of the women on the telly matter way too much to viewers.
Tell me, oh lady of advanced years, that you don't occasionally tut and scoff at what the female presenters and newsreaders are wearing. Or how they've done their hair, whether their roots are showing and their make-up looks like it's gone on with a trowel.
I do. We all do. While we're only half-listening to what they're saying, we're scrutinising every aspect of their appearance.
The BBC's bonfire is built on the vanities of us all.
That includes the endless stream of young lovelies who desperately want to break into the glamorous world of TV so they can combine their looks with their talent to become famous and earn lots of money while they can.
They shouldn't turn around and complain, 20 years down the line, that they don't like the rules of the game any more.
Got a view? Leave a comment below.
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