Chilli plants are usually bought in 6cm, 7.5cm or 10cm pots. Once the roots start protruding from the base of the pot the plants should be transplanted to at least a 15cm pot. Dwarf varieties can usually remain in the 15cm pot, but larger varieties will more than likely need to be potted on again in 3-4 weeks to a 20cm pot, or even larger.
Plants should be potted on using a good quality general purpose compost.
While plants can be grown in the soil outdoors, the soil temperature needs to be warm for them to do well, and typically a warm sunny windowsill, conservatory, or greenhouse will be required for the Irish climate.