Yes, under Regulation 49(2) any person who plants, disperses, allows or causes to disperse, spreads or otherwise causes to grow Japanese knotweed or any of the other invasive plants listed in the Third Schedule of the European Communities (Birds and Natural Habitats) Regulations, 2011 (S.I. No. 477 of 2011) shall be guilty of an offence. Furthermore, Sections 52(7) and (8) of the Wildlife Act, 1976, as amended, 1 make it an offence to plant or otherwise cause to grow in a wild state exotic species of plants.
The National Biodiversity Data centre has produced a frequently asked questions guide on Japanese knotweed in Ireland that is very useful.