Immigration:

If you are a UK or European citizen and your partner is from outside the European Union, civil partnership will entitle her and her dependent children to reside in the UK. If you were born in Northern Ireland and you fit the definition of a worker under European Union law, your non-European partner and dependent children will be entitled to benefits and tax credits immediately and generally ‘have recourse to public funds’, and your partner will be automatically allowed to work.

This stems from the fact that people born in Northern Ireland are entitled to Irish as well as British citizenship and a native of Northern Ireland can elect to be treated under UK immigration law as an Irish rather than a British citizen. It is a complicated issue and you should seek legal advice, but it is an important advantage; other UK citizens have to support their non-European partner and children without recourse to public funds for the first years, and the partner is not automatically entitled to work.

For full information on civil partnership and immigration go here.