Hi matchett,
Thank you very much for your speedy reply, and for the tip to look at Ballynahatty Church records.
The 1901 Census of Ireland lists a Rachel Houston living in Bundoran, Presbyterian, aged 76, making her income from land and houses, not married, born in Omagh, County Tyrone. If this is the same Rachel Houston, she would have been 10 when she made the sampler which would fit.
Out of interest, we also have a letter in our collection dated 19th March 1821 from Jacob Rodgers, Philadephia to Ross Huston [Houston?], Omagh. (link to transcription below) Jacob writes to Ross about how visiting Ross's sister. If it is the Ross within your family, do you know of any emigration stories within that generation?
http://nmni.com/uafp/Collections/Emigration/Letter-from-Philadelphia-to-Omagh,-County-TyroneOff to enquire about Church records!
Many thanks again for your help
Fiona