Before starting a search do check out the help file at http://registers.nli.ie/help. It has concise information on locating a parish and viewing a register. It will save you time.
While my Irish interest is limited I was interested to see if there's any trace of my elusive great-grandfather John Marmon listed as born in Kilkeel, County Down in the 1911 census of Wales.
Searching the parish name brings up information on three microfilms: 05477 / 05 with images for Kilkeel baptisms July 1839 to 12 Sep. 1877 and marriages 09 May 1839 to 18 Apr. 1876 in 115 images; 05477 / 07 "a modern transcript of the entries in the previous register for 1857 - 1878" in 169 images, and; 05477 / 06 with baptisms 03 May 1857 to 09 Aug. 1878 and marriages 30 Oct. 1867 to 19 Apr. 1869 in 46 images with the notation that "The entries are not the same as those in the previous register."
Before clicking on the appropriate microfilm scroll down to read You may be able to find more information on registers for this parish at: Irish Times Ancestors and/or Roots Ireland.
I've not had the chance to go completely through the microfilms, just to sample, but already found one entry of note.
The NLI is to be congratulated on making available this nice facility. The images are available in a convenient manner, short of name indexing. The quality of the originals leaves much to be desired, I found parts of pages missing in the Kilkeel register, you can't expect miracles - sorry, no sainthoods for NLI staff - just kudos.