Hi Simon,
No online database for births marriages and deaths in Ireland is complete -very far from it, but you're in luck with your two births

:
First with a very quick search I found these two below on the IG - It's always worth searching the LDS site (note the two different spellings of the surname of the two children):
Emma McCawley was born 26th June 1868 in Belfast to Thomas McCawley and Jane Simpson
Margaret Mcauley 28th April 1866 to the above parents
Next I had a quick look on ancestryIreland ( Ulster Historical Foundation).There is only one Emma McCawley listed with them on their site( It's in 1868 and spelled the same way as the IG and the same way as the below civil index info for her- so it looks as if its a very good possibility it is her if you want to pay to view it) . There is also a birth listed in the year of 1866 for a Margaret Mcauley which looks a good possibility too.
I then looked on Emerald Ancestors as they have some (not all) births and marriages from the civil indexes ( transcribed) and you're totally in luck there too for both of them (again note the same spellings - as it does look as if they are both registered under these same two different spellings in the civil indexes)
*Civil births:
Emma McCawley 26th June 1868, civil district Belfast, Sub district Belfast no 3 . Parents are Jane Simpson and Thomas McCawleyand
Margaret Mcauley,28th April 1866, civil district Belfast, Sub district Belfast No 5 . Parents are Jane Simpson and Thomas McauleyYou can either order the birth certificates for the two girls from either of the GRO's in Ireland by using the *above civil birth index info I've given you, or pay for a transcription from Family Ulster, or you can view the transcribed birth details at the UHF (ancestryIreland) for 6 pounds each for Emma and Margaret
The marriage:
I also quickly looked for a marriage. None on Emerald ancestors that I can find with a quick look , but there are quite few marriages on the ancestryIreland (UHF)site that I can see Jane simpsons marrying and Thomas
Mcauleys marrying in the same years.
Whether a Jane Simpson married a Thomas Mcauley is on there I do not know - it's difficult narrowing it down with their free search- The only way to tell if they are on there for sure is if you buy credits for one of the given names- either Thomas's or Jane's name and view those entries that you buy- It costs 6 pounds for 5 views of your name that you buy.
Another option to find the marriage in the civil indexes - if it is registered, is to pay Family Ulster for a look up in the indexes ( costs one pound a year on a name), but tell them about the two ways of the spelling of his name
Hope that is of some help
Kind regards

P.S These are the sites mentioned in this thread ,just in case you don't know about them:
Emerald ancestors for some civil index entries
http://emeraldancestors.com/ ancestryireland (The Ulster Historical foundation)
http://www.ancestryireland.com/index.phpFamily Ulster
http://familyulster.com/