Dear Karin
Welcome to Ireland Reaching Out!
As you can see from the link below, Tullyfern is a Civil Parish in Co. Donegal:
http://www.irishtimes.com/ancestor/fuses/townlands/index.cfm?fuseaction=TownlandsInCivil&civilparishid=795&civilparish=Tullyfern&citycounty=Donegal
Presbyterian records for Ramelton begin in 1806:
http://www.rootsweb.ancestry.com/~fianna/county/donegal/donpres.html
I assume that if you have been in touch with the church in Ramelton you were unable to find a marriage record for John Senior?s wife? I also assume that there was no mention of her on the children?s baptism records? Have you looked at Slater?s Directory to see if the brothers are mentioned as grain merchants? http://www.failteromhat.com/slater/0219.pdf
You could search the sites suggested below for the Clark family:
There are some places where you can search online, however these websites do not have complete collections and many do not cover the period after 1900:
www.familysearch.org has a huge database of genealogical records including some church records for Ireland.
www.rootsireland.ie have a large collection of Irish Church records, however you have to pay to use this site.
http://churchrecords.irishgenealogy.ie/churchrecords/ have begun a project to upload Irish church records to their site free of charge. However, so far only Counties Dublin, Kerry, Carlow and parts of Cork have been covered.
Other important sources include the Tithe Applotment Books and Griffith?s Valuation. Even though these sources only record the head of the household, the information is very important and can be valuable:
Tithe Applotment Books (1823-1837): http://titheapplotmentbooks.nationalarchives.ie/search/tab/index.jsp
Griffith?s Valuation (1847-1864): http://www.askaboutireland.ie/griffith-valuation/
The National Archives of Ireland: http://www.census.nationalarchives.ie/search/ you can search the 1901/1911 Census on the National Archives page too, for instances of the family name at that time
Remember to post any new information that you find here. The more information you post, the more likely it is that one of our volunteers will be able to advise or assist you. Also include information concerning which sources you may have already used so others may further your search.
Best of luck with your search
Clare Doyle
Genealogy Support