Is it possible that Christopher's baptism was on 26/4/1824?
There is a record accessible via Atavus search engine (search.atavus.org) in the record set "1829-1890 COI Magheracross Armstrong Vital Records Gw Armstrong" as follows:
Christopher Armstrong Ballinamallard not listed 26/4/1824 not listed Christopher Armstrong Mary
He may have been the Christopher within this family, all baptised at Ballinamallard :
9/6/1809 David
30/6/1811 Ann
12/12/1813 John
14/4/1819 Robert
26/4/1824 Christopher
5/4/1827 Thomas