Hi Amanda,
As promised, I got a quick look at the microfilm of 1st Dromara Pres. Church records today. There was no record of any Craig's in the early marriage records.
I checked baptisms from 1774 - 1786 & got the following:
[1777] "Willm Craig & Mary McMurry had a male child born Decr 5th Baptised 12 called Robert."
[1780] "Willm Craig & Mary McMurry had a male child born June 16 Baptised 21 called William."
This confirms the record in IGI for William, and gives you a brother & his parents!
I also checked baptisms from 1799 - 1805, but there was no record of any baptisms of William & Elizabeth's children. Although some of the records were very faint, I am sure that William wasn't there. (There were Craig baptisms - Robert Craig & Margaret Walker's children - possibly William's brother?)
Unfortunately I didn't have time to check further, but as at least 1 child was born/baptised about 1802 I think that it is unlikely that the baptisms were at this church. There were no Miller records in the years I searched.
I think IGI shows the birthplace as "Dromore", where there is also a Pres. Church. There are other churches in the area e.g. Drumgooland, but without knowing where they lived it will be difficult to find the baptisms/dates of birth. The absence of baptisms suggests that they either moved to another area, or just to another church.
I will check the Dromore records when I am next in PRONI, but I am not sure what date those records are from - lets hope we get lucky!
Hope this is useful - regards,
Eddie