You don’t say what denomination Katherine was but I’d guess Presbyterian. I assume Katherine’s family came from Killyleagh. Tradition was to marry in the bride’s church. If so, then the bad news is that Killyleagh 1st Presbyterian’s marriage records for the years 1757 to 1833 are lost and so you may not be able to find the marriage. However their baptisms survive for the years 1693 to 1757, so if that’s where Katherine was baptised you may be able to trace it. There’s a copy of the baptism records held in PRONI (the public record office) in Belfast. They are not on-line and a personal visit is probably required to access them.