1. Dear *****, your first step in apply to marry in Ireland is that you both must provide a Notification to the Registrar of Marriages in Ireland or your intention to marry. This must be done at least three months before your date of intended marriage. It is only after you have received an acknowledgement of your intention to marry from the Registrar that you will then be apply to apply for a visa for your girlfriend to come to Ireland to celebrate the marriage.
2. Here is a link to the page in the website of the Irish naturalisation & Immigration Service which sets out the requirements for apply for a visa for your girlfriend in order that you get married in Ireland http://www.inis.gov.ie/en/INIS/Pages/Marriage%20Civil%20partnership%20Visa
3. You will see that there are quite a few requirements. You will not need a visa as Ireland and Britain operate a common free travel area and citizens of Ireland and Britain can freely move between the two countries.
4. Here is a link which sets out the Notification requirements which are required for an Irish marriage http://www.citizensinformation.ie/en/birth_family_relationships/getting_married/notification_requirements_for_marriage.html
5. Please Rate the answer as unless you Rate the answer your Expert will receive no payment for answering your question.