As long as care is taken over surname variants, and the names of both parties are known, research in the marriage indexes is straightforward. If two people married each other then obviously the registration district, volume and page-number references for them in the indexes must be the same. Therefore it is only necessary to cross-check the two names in the indexes, working back from the approximate date of birth of the eldest child if this is known, until two entries are found in which all three references correspond. This process is slightly more cumbersome with the FamilySearch online transcripts but still perfectly possible. Marriage records are especially important in the early years of civil registration, since they record the names of the fathers of people born c.1820 to c.1840, as well as their approximate ages, thus providing evidence that can be used to establish earlier generations in parish records. For non-Roman Catholic families the value of these records is even greater, as the records of non-Roman Catholic marriages start in 1845.
The full set of General Register Office historic marriage records is free onlline at IrishGenealogy.ie. These are:
Using marriage records with other sources
The 1911 census records the number of years a couple have been married, the number of children born, and the number of those children still living. This information is obviously very useful in narrowing the range of years to be searched for a particular marriage. In the case of names which are common in a particular area, the fathers' names supplied in the marriage record are often the only firm evidence with which to identify the relevant baptismal record in the parish registers. Once a marriage has been located in civil records, thus showing the relevant parish, it is always worthwhile to check the church record of the same marriage. As church marriage registers were standardised, from the 1860s on, they became more informative, in many cases supplying the names, addresses and occupations of both the mother and father of the parties marrying. In the case of most Dublin Catholic parishes, this information is recorded from around 1856.