The governance of inter State broadcasts in the European Union is conducted under the terms of the Audiovisual Media Services Directive (or AVMSD for short), which is the latest iteration of a long standing series of such Directives, all of which are premised on the country of origin principle. The Directive also sets down a series of restrictions that apply to television broadcasts across the EU, including issues around types of advertising, hate speech and sponsorship of certain types of programmes.
Under the ‘country of origin’ principle, broadcasts are regulated in the country from which they originate, with provision made in Irish law that these network providers must carry Irish terrestrial channels. However, once these provisions are met, commercial television suppliers based in other countries can offer those channels they wish to, depending of course on commercial agreements with the channel owners themselves. I gather that some commercial television providers choose not to make certain terrestrial channels readily available on their Electronic Programme Guide (or EPG); this is of course a matter for them.