Thank you for the replies! They confirm the information that we have, which is very useful. My grandfather's birth certificate (certified copy) and the registry page refer to registration in the district of Galgorm, which I misread as the birthplace, which, as rightly pointed out, was actually Kells.
The Belfast civil marriage register entry for John and Sarah Jane - that matches in every other detail - shows John as John Forster, and his father as William Forster; the only place where this spelling occurs (so far). It has been suggested that the spellings were interchangeable at one time. Is that likely to be the case here?
The death registers (from Scotland's People) show that John Foster (senior) died in Govan, Glasgow in 1884, and his wife Sarah died there in 1886. John's parents were William Foster (recorded as 'linen manufacturer', which I think means 'weaver'!) and Sarah Kenney. Sarah's parents were William Anderson and Maria Townsley.
The 1881 Scottish census records John and Sarah with three sons, James, Robert and John, in Govan, but we have found no record of William in Scotland. Perhaps he died in infancy, or had already left home before the rest of the family moved to Scotland. So far, we don't know.
Thanks to all who are taking an interest in this - we much appreciate your knowledge and effort.