How and When do I need to fertilise my Iris?

The short answer : one month before bloom season, so about the time your tulips are blooming. Then again in September after flowering. So there you go, easy really.
I recommend bone meal or super-phosphate and a light balanced fertilizer such as 10-10-10 or 6-10-10 depending on the amount of nitrogen in your soil. Do not use anything high in nitrogen as nitrogen promotes rot problems. Most fertilisers are based on the three major plant nutrients: Nitrogen (N): For green leafy growth. Phosphorus (P): For healthy root and shoot growth. Potassium (K): For flowering, fruiting and general hardiness. The N:P:K ratio is usually quoted on the product packaging. I recommend that you test your soil as your soil may already be high in nitrogen. Kits are readily available from DIY centres and on-line.
Keep the fertilizer 5-10 cm away from the rhizomes.
To get the best results fertilise in moderation. The only thing Iris resent more than underfeeding is overfeeding.
For Dwarf Iris (that can flower as early as March) apply fertiliser in mid to late February.
Intermediate and Tall Bearded Iris in intervals coinciding with the month before their bloom cycles.
Should I fertilise Iris when I plant them, and with what?
At planting, sprinkle a teaspoon of a low-nitrogen fertilizer around the newly planted rhizome.
If you are preparing a large area for iris planting, incorporate 30g (2 tablespoons) per meter square to the garden bed.
Should I fertilise Iris after they flower?
Wait a month or so after blooms have finished, I like to fertilise in September, around my birthday (cards and gifts welcomed).
This gives the plants enough time before winter to take up the nutrients they need going into their winter dormancy.