Heh, I'm 43 and got carded just a few days ago, while waiting for my hubby to park the car (he's only two years older than me but a distinguished grey now, so when I am with him, it's much less likely I get carded).
I'm also 5'0" and "cursed" with a baby face. I put "cursed" in quotes, because when i was your age, I really did feel like it was a curse. And, I know you are going to hate hearing this, because everyone probably already tells you this. I am just saying that I know exactly how you feel, since I've been there.
I can promise you this: *someday* - it may take 20 years from now - you are going to finally learn to love being carded and mistaken for younger than your age. Someday. Yeah, 20 years is a long way to wait for that!
Meanwhile, my survival tips are:
1) Don't take up smoking. I thought it made me look older, it doesn't, it just made me look silly (I'm nearly 3 years quit now... but smoked for over 20 years and that was really stupid.... and it didn't even give me distinguished wrinkles either!).
2) Don't wear ultra-high-heels. It will ruin your calf muscles, possibly your back too. It's okay to wear flats even if you are barely 5'0". Baby your feet and they will thank you later. Bring out the high heels for special occasions only. I do think (in retrospect, having gone through this "4 inch heels make me look older/taller" stage" myself - that ultra-high-heels actually have the OPPOSITE effect on us short people. A 1-1/2 to 2 inch heel is great and streamlining. Leave the higher heights for people who have the overall height.
3) Focus on jackets. They give an air of authority. Needn't be boring and conservative - funky fun age-appropriate jackets are fine. The secret of jackets is that it gives you broader shoulders and more "presence."
4) Spend the $ for perfect fit - petite clothing lines, tailoring if you must. Poorly fitting clothing looks bad on everyone, but nothing makes you look "too" young than wearing clothing that is too big and overwhelming.
5) Go for quality over quantity. This gives you an aura of being your age - or older - despite how short or baby-faced.
6) Nix the teen clothing lines (even if they fit). Or use them very judiciously. Do not dress head-to-toe in teen-appropriate clothing. (I don't know the brands in Ireland, but for example, even if I can fit quite well in Aeropostale and other teen lines my teen daughters happen to wear ... I don't wear them because then I'd look like a teen!
7) Agree that shorter hair may make you look more your age. Or at least, just having a style, not an "unstyle." If that makes sense?
8) No more tee shirts. Sorry! Go for quality knit tops instead. Or, wear your tees but with a top-notch, great fitting, structured jacket over top.
Hope that helps!!!