They are 18 or above on the day of their marriage (consent must be obtained for those aged between 16 and 18)
They are not related to one another in a way that would prevent them from getting married
Neither of them is already a civil partner or is legally married
They are not of the same sex
They are capable of understanding the nature of a marriage ceremony and consenting to marry

The types of marriage ceremony are:

Civil marriage in the Registrar's office in Antrim Civic Centre or in an approved venue (subject to the availability of the Registrar)
Religious marriage

Before a marriage can take place, both parties must "Give Notice of Marriage" by completing a Marriage Notice Form and returning it to the Registrar, along with Full Birth Certificate, Passport and Decree Absolute / Spouse Death Certificate if previously married and fee (see Religious / Civil Marrigae link) . If the mariage is to take place in Antrim, but you live in Belfast, you will need to submit your Marriage Notice Form to Antrim Civic Centre. Information from the form helps the Registrar collect information about your ceremony and prepare for your details for registration. A Marriage Notice Form lasts for 12 months from the date of notice.