It is an offence to obstruct and angler who is fishing lawfully.

As per the Fisheries (Consolidation) Act 1959 Part XV11, Section 287

‘If any person obstructs any person lawfully engaged in fishing or in proceeding to or in returning from lawful fishing, such first-mentioned person shall be guilty of an offence under this section and shall be liable on summary conviction thereof to a fine not exceeding five pounds.’
(Hyper link: http://www.irishstatutebook.ie/1959/en/act/pub/0014/sec0287.html#sec287)