The first thing you have to establish is which church your ancestors attended. Presbyterians in Ireland didn’t use the parish system (which normally indicates where a person will attend church) and their choice was based mostly on personal preference. So the nearest Presbyterian church might not necessarily be the one they attended. And in places where there were lots of Presbyterians they often built additional churches near to each other.
I don’t think any of the Presbyterian records for Co Monaghan are on-line. However they have been copied on to microfilms that can be viewed in PRONI Belfast. Obviously if you can’t get to PRONI you would need to arrange for someone to look them up for you. The following (taken from the PRONI website) tells you of some of the Presbyterian churches in the area. Alternatively you could try writing to the churches, but they do get a lot of genealogical requests and I can’t guarantee you’ll necessarily get a reply.