Marks and Spenser is your man, River Island if you are on a tighter budget both have great looking suits but the marks and sparks suit will last longer.
Suitable company on the corner of Stephen street and drury street do great deals on designer suits,.
Last but not least is arnotts, you;re sure to pick up something decent in any on of the shops above
Also, if you;re looking for a new short try TM lewin just off Grafton street, across form Keoghs. They're doing shorts for €40 or 4 for €119