PRONI is the Public Record Office of Northern Ireland. It is the official archive of Northern Ireland, and the official place to deposit public records in Northern Ireland. It is located in Belfast, and is open to anyone who wants to come in and search through the archives.
It is free of charge, and you don't have to make an appointment. There are millions of documents in this archive, dating from 1600 to the present day. The public records are from 1921 to present day, and there are some records from the nineteenth century and late eighteenth century. If your genealogy research has led you to ancestors who had Irish ancestry, this is an excellent archive to search through.
Recently, PRONI made an amazing archive of historic family photographs available for viewing through Flickr. This is extremely convenient if you wanted to view these photos, but don't happen to live in Northern Ireland, (and don't really want to search the PRONI website itself). Flickr is a free photo-sharing website. People make an account, and add whatever photos they want to the site. Anyone at all can then view the photos that are located on Flickr.
The photos that appear on Flickr are wedding photos that were taken by the H. Allison & Company Photographers. The PRONI Flickr page is separated alphabetically. So, for example, if you are looking for photos that are of a surname that starts with A or B, then you want the first album. There is an album for surnames that started with Mc, as well. Each photo has notes attached to it on Flickr that provide information like the family name, the day, month, and year the photo was taken, and the size of the original format of the plate glass negative that the Flickr image came from.
The wedding photo collection covers over fifty years of wedding and family portraits, and were taken between the year 1900 and the year 1952. There are about 200 images in all right now, but they are planning on eventually adding all 1530 images as they convert them into digital format.