Leaf celery, or herb celery, is far easier to grow than regular celery and provides an abundance of leafy growth with the same flavour. I think of it as cut and come again celery. An early spring and mid summer sowing should give you leaves all year round in Ireland as they are very winter hardy.
Sprinkle a pinch of the tiny seeds on a tray filled with fine seed compost. Don’t bother cover them up as they need light to germinate. They will germinate over a few weeks. When big enough, prick them out into pots to grow on before planting out. Being close to their wild relative, they are unfussy and will be happy anywhere sunny and reasonably fertile. Apium graveolens