This is mainly because, after a few early hiccups, Anglican, Presbyterian and civil officials were issued with printed registers (books); having recorded the details of the ceremonies they had carried out in the register books, they would transcribe the details into printed forms (see example of a blank marriage register form here) and submit these transcriptions to the local Registrar.
The registers - the originals - were handed over to the local Registrar for safe-keeping when they ran out of room for more entries. These are generally referred to as 'the locally-held [civil] registers'.
(Unfortunately, RC priests did not use the pre-printed registers forms when they first joined the compulsory registration system in 1864; the responsibility was given to the bride and groom to register their marriage, and some clearly had other things on their minds. From 1880, however, the responsibility for registering RC marriages was given to the parish priest.)
Each quarter, the Superintendent Registrar gathered in all the transcribed forms from his district's clergy. These were then sent to the General Register Office in Dublin where they were sorted into alphabetical parish order, then into Poor Law Union order.
Bundled together, these loose forms were then assigned a volume number and each page numbered, creating a national index of marriages for each quarter of the year (later indexing was done annually).
Inevitably, mistakes were occasionally made during the administrative process and although relatively rare, genealogists have come across cases of Irish marriage records being omitted from the national index even though the marriage was correctly issued and recorded in the locally-held register.
The first stop for Irish marriage records is usually the civil registration index. These are compiled and maintained by GRO in Roscommon and, since 1922, by GRONI in Belfast. Until recently, they were available only in the research rooms of those organisations.
In the last couple of years, a number of websites have begun to offer transcripts and images of Ireland's historical civil registration records. Some of these offer free access. Some don't. Some are nearly complete; most are not.
GRONI's online Geni database is complete for Northern Ireland from 1845/1864 to 1943. The best database for the rest of the island is IrishGenealogy.ie, which will finish (fingers crossed) its upload of free, indexed, register images in Autumn 2018.
There's an overview of how to use the indexes to obtain copies of marriage register entries (which provide the details for marriage certificates) on the main Irish civil registration page.
Irish family historians don't need to cough up outrageous fees for a legal copy of their ancestor's marriage certificate. You can get one, sure, if that's really what you want - follow the instructions at certificates.ie.
But most genealogists don't need a certificate. They need only the information contained on a copy of the register entry. Online images of the register entries from 1878 to 1921 for the entire island (including Northern Ireland) and for 1922-1943 for the Republic of Ireland can be viewed free at IrishGenealogy.ie.
For online images of the register entries from 1845 to 1943 for Northern Ireland there is a small charge to view them at GRONI's Geni database.
The General Register Office (GRO) in Roscommon provides what they call 'Research Copies' viz. photo copies of the register entries for marriages that took place from 1845 to 1921 for the entire island and for 1922 to the present for the Republic of Ireland. They cost €4 - €6.
The examples of marriage records shown below are Research Copies.
images below are a selection of Irish marriage records obtained for my own family history research. In each case the image shows