If you've been in touch with the Presbyterian Historical Society, then you've probably been dealing with Valerie Adams. She's pretty knowledgable in my experience. However according to the website below, Tullyallen is all in Co Louth except for a small part in Co Meath. However Parish boundaries do sometimes move. Louth and Armagh do have a shared boundary, so it's possible that at one time part of Tullyallen parish extended into Armagh. However most of the parish seems to me always have been in Louth, so that's where I'd focus any search. There are not many Presbyterians in Louth these days and whether records from a secession congregation there c 1800 would have survived is doubtful. But you never know.