The National Library of Ireland on Kildare Street Dublin holds the church registers, but not civil records of births, marriages, and deaths. For civil registration records, you can go over to the GRO, 3rd Floor, Block 7, Irish Life Centre, Lower Abbey Street, in Dublin, which has research facilities, where you can purchase the certificates you are interested in. For more information about the GRO offices in Dublin, go to: http://freepages.genealogy.rootsweb.ancestry.com/~irishances...
The other option you have, which would free you up for more research time at the National Library, is to send application forms to the GRO in Roscommon Town, County Roscommon, for the civil birth, marriage, and or death records you are interested in. The GRO website for Roscommon can be found at: http://www.groireland.ie/
Civil registration in Ireland began in 1845, when the government recorded Protestant marriages only. Registration of births, marriages, and deaths for all religious denominations in Ireland commenced in the year 1864, which means there won’t be any civil birth, marriage, or death records for your Catholic ancestors until after 1863. Before 1864 you’ll have to locate church baptism and marriage records for your ancestors.
Best of Luck,