The simplest method is to order a copy of the register from GRO Ireland in Roscommon. Research copies can (as of March 2016) be obtained for €4. I do not believe there is currently the facility to order research copies online, which means you have to send in the form by post or fax, although they can email you the copy. Instructions and a link to the form can be found on the Irish Genealogy website.
As an alternative, for births (1864-1881, 1900-1913, 1930-1955), marriages (1845-1870) and deaths (1864-1870) you can order the relevant film from FamilySearch. Registers in Ireland are not under the same regulations as in England and Wales meaning it is possible to view the registers directly in this way without ordering a certificate. This would only be cost effective if you had multiple events you wanted to lookup on the same film. The relevant sets of films to order, based on the volume number you found in the index, are:
- Quarterly returns of births in Ireland, 1864-1955, with index to births, 1864-1921
- Marriage records, 1845-1870, with indexes to marriages, 1845-1921, in the General Registry Office of Ireland
- Death records of Ireland, 1864-1870, with index of deaths, 1864-1921