You could well be right. Townland of Tullyvallan in the parish of Newtown Hamilton. Griffiths Valuation has Anne, Samuel & James Stephenson listed as living there in 1864. Still Stephensons there in 1901:
I see there’s a Reformed Presbyterian Church there, known as the secession meeting house. That might have been the congregation they attended. According to the PRONI website the earliest surviving records for that church date from 1820 (a list of members).