I've made a firm decision" says the supremely irritating Andie MacDowell in L'Oréal's new Revalift TV ad spots. Using the meagre weapons in her arsenal, which include wooden acting skills and a complete lack of facial expression, she tries to convince hapless civilians that the average 51-year-old woman can look as good as she does. And it's apparently all down to relying on a pot of face cream she picked up for a few quid when she was down in Tesco getting the messages.

Oh yeah, that's right: it's pretty unlikely that Ms MacDowell has been on the business end of a shopping trolley at any time in the recent past. The question is, do we really believe that A-listers use the products they endorse on their celebrity skin?

Sadly for beauty companies, the answer seems to be a firm no. We asked on Beaut.ie and the response was one of overwhelming scorn. "I really can't see Girls Aloud washing their hair in the morning with Sunsilk, then applying their own-brand eyelashes, then snacking on a KitKat Senses whilst having a wee go on their Nintendo DS," commented reader Gemma, who added, "there really wouldn't be enough hours in the day to use all the products they endorse!"

But if celebrities aren't using the brands whose cash they're trousering, what do they slap on? Through a combination of super-sleuthing and plain old-fashioned investigative journalism (ahem), I've come up with the goods on who wears what.

SOPHIE ELLIS-BEXTOR

Beloved of gals who like to wear their make-up a little more creatively, Ms Ellis-Bextor is the face of Rimmel's Moisture Renew lipsticks. Oh, so that must mean she wears the rest of the products then? Er, no. Soph's favourite foundation is by Nars, and if you'd like to copy her alabaster look, then head to Brown Thomas to bag a bottle of Sheer Matte Foundation (1) in Mont Blanc, €34.

GIRLS ALOUD

Posh Prescriptives is a fave of the girl group, who are all said to be fond of their Soft-lining pencil (2) in Jet, €18. It's smudgy, it's creamy and it's just right for those OTT smokey eyes the quintet rock on stage. Line up for it (geddit?) at Brown Thomas and House of Fraser, Dundrum.

SCARLETT JOHANSSON

Scarlett is the face of Dolce & Gabbana's new luxe make-up line and the ad spots picture her in all her Marilyn Monroe-like, blonde-bombshell glory. Red lips and porcelain skin are the order of the day for D&G, but Scarlett actually uses cult brand Lipstick Queen (3) on her days off. Find the line at Space NK, where the gorgeously formulated reds will set you back €18.

BLAKE LIVELY

The Gossip Girl glamazon is slowly stealing the show from Queen B with those legs that go on for miles and that high-octane boho look. Her make-up on set is usually peachy perfect, but in her private life, Blake Lively is a fan of Urban Decay's 24/7 Glide-on eye pencils (4). They're €14 and you'll find them at department stores such as the Arnotts Project. Psst: Rihanna and Ladyhawke are fans too.

HILARY SWANK

Despite being the face of rival brand Guerlain's Insolence fragrance, the razor-cheeked Million Dollar Baby star emphasises those prominent bones with Yves Saint Laurent's Touché Blush (5), €40. It's a cinch to use, so she'd probably be able to manage it even with boxing gloves firmly attached.

LADY GAGA

I had Gaga down as a Barry M/Homer Simpson make-up Gun kinda gal, but it turns out that she's got posher pretensions altogether and Dior is a favourite. In fact, myself and the Lady have something in common: we both love the brand's Five Couleur Designer (6) in Navy Design, €53.50, at department stores.

PEARL LOWE

As mum to model-turned-IT-girl Daisy and wife of Supergrass's Danny Goffey, I bet ex-rock chick Pearl Lowe's had a few sleepless nights in her time. Origins to the rescue! Lowe is said to be a fan of the Dr Andrew Weil for Origins Bedtime Spray (7), €31, which she uses pre-snooze to promote a calm and restful sleep. Aww.

LAUREN LAVERNE

I love the lovely Lauren, with her razor-sharp wit, ex-Kenickie-band-member status and peachy skin. And that peachy skin is achieved through use of Clinique's Anti Blemish 3 Step System (8), €45, apparently. Products to cleanse, exfoliate and moisturise keep Ms Laverne's skin looking fab for all those telly appearances.

RACHEL STEVENS

Not normally on my radar, Stevens has been counting column inches again with her recent marriage, and before that, the shock ring-stealing incident in which her diamond engagement band was stolen from her finger by muggers with taste. I judge her newsworthy for another 15 minutes, so I'm wondering what she uses on her lovely clear skin. Gallic brand Caudalie is the answer, and in particular, their Beauty Elixir (9), €35, at Space NK.

RIHANNA

Not one to do anything by halves, recently Rihanna's quiff has been reaching skyscraper proportions and her eyelashes have been following suit. She's a fan of bargaintastic Ardell falsies (10), which come in all manner of shapes and sizes. A recent US import, they're also very recession-friendly and will set you back a mere €5 at Boots.

- Kirstie McDermott