If you come across a marriage record which notes the granting of a dispensation of consanguinity, you should definitely sit up and take note: you are looking at evidence of a common ancestor (or a pair of common ancestors) shared by both bride and groom. However, as Dan MacDonald points out in his Marriage Dispensations in Roman Catholic Marriage Records, the presence of a dispensation does not necessarily imply that a couple were related. It depends on the type of dispensation.
In addition to dispensations of consanguinity and affinity (which indicate a blood or marital relation, respectively, and which are pretty much always of interest to the genealogical researcher), the Church also granted dispensations from certain established rules and procedures surrounding the marriage ceremony.
For example, when John Killeen married Margaret Fahey on 20 December 1852, the priest (Rev. M. Molloy) noted that he had obtained a dispensation from the Bishop of Bytown to perform the marriage ceremony at “a fordidden time.” The “forbidden time” in this case was that of Advent (from the start of Advent to the Feast of the Epiphany); another “forbidden time” would be that of Lent (from Ash Wednesday to Low Sunday, or the first Sunday after Easter).
In 19th-century Ottawa Valley area RC parish registers (and no doubt in the RC registers of many other places too), the most common dispensation was that of a dispensation of one or two (and sometimes, although less frequently, of all three) of the required banns.
Basically, before a couple could marry, the Church required the publication of their marriage banns on three consecutive Sundays. And for nineteenth-century rural parishes, needless to say, we’re not talking about the printing of a notice in the parish bulletin! (they didn’t yet have parish bulletins, of course; and many, if not the majority of parishioners wouldn’t have been able to read a bulletin in any case). “Publish” meant to make public, as in, to read from the pulpit at the Sunday mass. But when people belonged to missions, and not yet to regular parishes, and were served by travelling missionary priests rather than by stationary and regularly salaried curés, many places did not yet offer masses on three consecutive Sundays, from which the banns might be read. Moreover, often one or both parties to the marriage lived too far from the church or mission to be reasonably expected to attend mass on a regular (three consecutive Sundays, say) basis. So dispensations of banns were fairly common in early Ottawa Valley RC marriages, and were often granted by the parish or missionary priest himself (rather than by a higher-up such as a bishop).
Here’s a marriage record where the couple received not one but two different dispensations. When George Cahill married Ann Shirley (25 January 1886), the couple received a dispensation of two banns, along with a dispensation of consanguinity in the third degree:
The above record refers to one ban having been read during the homily at the parochial mass, with a dispensation for the two other bans having been granted by the undersigned priest; and also (and more interestingly, in genealogical terms) to a dispensation “du troisième au troisième degré de consanguinité” having been granted by the Vicar Apostolic of Pontiac.
Note that while it was the undersigned priest (J.G. [Arthur?] Ouellet) who granted the dispensation from the publication of two bans (with one ban having been read at the parochial mass), the dispensation for consanguinity had to come from higher up in the Church hierarchy: in this case, from Narcisse Zéphirin Lorrain, Vicar Apostolic of Pontiac (and later Bishop of Pembroke).
Since dispensations of banns were extremely common in the early register for the mission/parish of Ste. Anne, this dispensation, in and of itself, is unremarkable, and suggests no necessary blood or marital relation between bride and groom.
The dispensation from the impediment of consanguinity, on the other hand, is genealogically noteworthy: it suggests that George Cahill and Anne Shirley were second cousins. “Du troisième au troisième degré de consanguinité” (of the third to the third degree of consanguinity) means that both bride and groom were separated from a common ancestor by three generations: i.e., that they shared a common great-grandparent (or pair of great-grandparents). But note that it would also be possible to have something like “du deuxième au troisième,” where one party was two generations and the other party three generations away from a common ancestor (or pair of ancestors) [= first cousins once removed].
And just to make matters a bit complicated (but that’s half the fun of doing genealogy, after all): as best I make out, George Cahill and Anne Shirley were not second cousins, but rather first cousins once removed. Anne Shirley, daughter of Thomas Shirley and Honora McGuire, was the granddaughter of James Shirley and Catherine Butler (see Anne Shirley’s ancestry here). George Cahill, son of George Cahill and Mary Moran, was the grandson of Anne Shirley (m. Michael Cahill), and the great-grandson of James Shirley and Catherine Butler (see George Cahill’s ancestry here). They shared the same pair of ancestors (James Shirley and Catherine Butler), but Anne Shirley was two generations, and George Cahill three generations away from that common pair. In which case, the dispensation should have been of 2 to 3 rather than of 3 to 3 degrees of consanguinity.
But am I absolutely certain that I have correctly calculated the relationship between bride and groom in the above example? No, not at all. The early Canadian records are patchy at best; and the relevant Irish records worse than patchy (non-existent, really): so: I may be missing an important piece of information. However, the dispensation for consanguinity certainly confirms some sort of blood relationship (some degree of cousinship, that is, whether second cousin or first cousin once removed) between Anne Shirley and George Cahill.
In its discussion of “Consanguinity (in Canon Law),” The Catholic Encyclopedia has a handy little Genealogical table (scroll down to the Table of Consanguinity). Also see Dan MacDonald’s Worksheet For Tracing Dispensations of Consanguinity; and Jacques Louvel’s Demandes de dispense pour marriage.