The glamorous mum-of-eight was the subject of a 'happy birthday' hoax online after a popular website stated she had celebrated her 50th birthday last month.
The encyclopaedia website Wikipedia claims Miriam was born on January 6, 1960.
But the good-natured Prime Time presenter exclusively told the Herald that although it's not true, she isn't the slightest bit bothered about what age people think she is.
"I haven't hit that one quite yet but I promise to come back to you when I do," she quipped.
The statuesque blonde, who has just confirmed that her summertime chat show, Saturday Night with Miriam, is set to return for its fifth season later this year, has regularly been voted among Ireland's sexiest women on TV.
The down-to-earth presenter has always declined to reveal her exact date of birth.
"I don't think age matters. I think it is about being happy. Everybody should be thanking God that they are alive another day and not regretting it."
A source said Miriam only turned 49 very recently and so won't be celebrating her 50th birthday until next year.
The yummy mummy said she has no real secrets to looking good as she gets older, but said "being happy, hard work and good genes" definitely help.
Although she is one of the most stylish women on TV, she has always insisted she will never undergo cosmetic surgery.
"I dye my hair, I wear make-up, I've had my teeth whitened, I spend money on face creams. I'm proactive and I want to go on like that, but I won't go under the knife, because I think it makes everyone look like they've been through a wind tunnel," she said.
Meanwhile, Miriam's busy household did celebrate a different birthday with a bash recently. The presenter's youngest son, James -- or "Sweet Baby James" as she dotingly calls him -- celebrated his fourth birthday last weekend.
Miriam ensured she made time to throw him a special bash despite having an extra busy workload of late.
In past weeks she has hit the roads as part of an investigative report on the state of mental health in this country.
Speaking about the programme, Miriam said: "I went out on the road and have made a film on the shocking conditions in our psychiatric hospitals where people are locked away for years.
"We got permission to go to Grangegorman and film. It's genuinely very interesting and compelling TV."