The online 1911 census is only useful at the moment for people from Dublin.
A marriage cert gives both parties' fathers' names and the addresses the two marrying lived at at the time of marriage. It is easier to find a marriage cert because you have 2 people's name and you can cross-reference in the index. You need to know the year (or at least an approximate period) to search the indexes. If your grandparents came from Dublin, then start with the census. If not from Dublin, go to the national archives in Bishop Place in Dublin and search for marriage records for the relevant period. If you were to search for a grandparent marriage, this would likely yield an address to check the microfilm census records for 1911/1901, again in the National Archives. Their website gives all the details of how to apply for a reader's ticket and the staff there are very helpful.
Best of luck!