I buy most, if not all my clothes at allsaints (allsaints.com). It's a bit pricey but I find it worth the cost.
I buy Diesel jeans. You can get them in various shops in Dublin or if you know your size you can order off asos.com. Asos is good as it basically has every size combination, even the ones you won't get in shops (such as 30 waist with 34 leg). Again they can be pricey but they'll last ages and if you're like me you'll probably be wearing them most days anyway. They do go on sale though from time to time so keep your eyes peeled.
If I'm looking a cheapy runaround type stuff topman is grand. The only problem with topman is you have to sift through all the crap to find something decent. Topman do decent chino type trousers if you're into those.
The tricks are to buy stuff that you can wear both smart and casually. I have t-shirts that look casual when I wear them under a hoody but smart when I wear them under my leather for example. Or a cardigan that looks smart with a shirt under it but casual when a t-shirt is worn under.
And buy stuff that fits!
You should see your body shape, the clothes shouldn't be hanging off you. No point buying a load of nice clothes in the wrong size as you're wasting your money.