Irish political parties register at a national, not a local level, regardless of their presence or absence in various constituencies.  You could require a party to run a candidate in more than one constituency, but they generally do in any case. As a candidate can run as an independent in any case, party membership doesn't confer anything other than an organisational benefit in any case.  (Independents can form an ad hoc technical group in the Dail to get additional parliamentary rights/back-up staff etc.)

I'm not suggesting any restrictions on independents/small regional parties in national elections - just for the EP election to focus the campaigns on transnational issues.  It need not be a legislative bar - perhaps just some public funding for cross-national parties/campaigns.  I'm tossing this out as an idea - I'm not entirely sure whether it is a good one or not.

notes from no w here