It's the reference number you need in the Irish civil indexes.
Go to the GRO Roscommon website: www.groireland.ie/
Download the application form you need, and complete with the information you have, including the location, year, quarter (if one is shown), Volume & page number. Put that information on the lines for date and place of the event etc. Don’t worry if you have to leave some boxes blank, eg PPS number. Be sure to tick the box marked photocopy, if that suits you, as it’s cheaper. You then either fax or post the form off (there is no e-mail option for certs pre 1.1.1922). Their turnaround time is 7 to 10 days plus however long the post takes.