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

Given that it is hard enough to motivate people to vote once, I doubt there are many who would bother travelling and voting several times!

Nevertheless, for these to be considered truly European elections, you would expect their to be a European voter registration process which controls for duplicates and allows people to transfer from one country to another if they move residence, or even to vote in their home country election whist temporarily abroad in another member state.  (I don't mean just a postal vote, but physically voting in one country and having that vote counted in another (their home) country.

Ultimately an internet based voting system might be best, but how you would do that without very intrusive security measures would be important.  Perhaps a passport reader attached to a secure networked PC would be the answer.

notes from no w here