Presbyterians don't always attend the nearest church and some families have been known to change churches over the years so you sometimes have to check more than one place. Presbyterian congregations have also been known to split and form another church or join a new presbytery which can also make things more confusing when searching for family records.
For Magherafelt-
Magherafelt 1st Presbyterian Church- baptisms 1703-1706, 1771-1780, 1813-1861, 1889- ; marriages 1769-1782, 1845- . Originally part of Moneymore. Records on microfilm in PRONI, Belfast, and Coleraine Library, may also be available to order from your local LDS library.
Magherafelt (Union Road) Presbyterian Church- baptisms from 1868 (start of congregation).
If you know a location for the family it will be easier to see what other possibilities there are. There is lots of information in RESOURCES at the tiop of this board, including a section for Church Records and Graveyards. This link has map of Presbyterian congregations-
http://historyfromheadstones.com/index.php?londonderryYour ancestors may have been linen weavers in Ireland but probably turned to carpet weaving in the U.S.