It varies from country to country.
In Republic of Ireland, you can apply in person or by post (see GRO Ireland for details). Or you can apply online at Irish Certificates. You can order Irish long form birth, marriage and death certificates at Irish Certificates.
In Northern Ireland, you can apply for certificates to GRO Northern Ireland.
In England and Wales, you can get certificates from GRO UK.
In the United States, you can order certificates through VitalChek.
Regards
Genealogy Gene
In Republic of Ireland, you can apply in person or by post (see GRO Ireland for details). Or you can apply online at Irish Certificates. You can order Irish long form birth, marriage and death certificates at Irish Certificates.
In Northern Ireland, you can apply for certificates to GRO Northern Ireland.
In England and Wales, you can get certificates from GRO UK.
In the United States, you can order certificates through VitalChek.
Regards
Genealogy Gene