On checking the collection on my desktop pc this morning, I wasn't sure it offered non-FHC researchers anything other than an always welcome additional (partial) index of the civil registration records. However, on closer examination, it does.
While we await the upload of GRO's pre-1882 marriage and pre-1891 death register images to IrishGenealogy.ie, this collection is worth checking out, especially for pre-1870 marriages. It holds 32,997 marriage records for this period (irishgenealogy.ie has 790,000+) and they provide more information than we're accustomed to from an index. Here's an example: the 1851 marriage of James Adair.
On IrishGenealogy.ie, a search for this marriage would give you this result:
A search on FamilySearch's Ireland Civil Registration Indexes 1845-1958 would provide the same information, while searches on Ancestry and FindMyPast would provide the probable or possible name(s) of the spouse.
On FamilySearch's new index, however, this is the result you'd receive:
Not only is the spouse's name confirmed, but the names of James' father and the bride's father are also given. Usually you won't get that level of detail without sight of the marriage certificate.
Since the wait for IrishGenealogy.ie to add the missing images seems to be going on and on (despite assurances at the turn of the year that they would be uploaded 'soon'), researchers should take a dip into this index if the dates match the events they seek. It will be a fruitful and lucky dip for some, if not most.
UPDATE, 27 June: Thanks to Steven Smyrl MAGI for getting to the source of this 'new' collection. It seems these civil records have been extracted from existing mixed-source collections already available on FamilySearch: Ireland Births and Baptisms, 1620-1881, Ireland Marriages, 1619-1898, and Ireland Deaths, 1864-1870. So they're not new or enhanced records. They're what the pr/marketing bods call 'repurposed' – similar or partial product, different packaging.