Forsythes and Elvins

My great great grandfather William Forsythe was Born in Castlederg, County Tyrone in 1824. His father was Samuel Forsythe. He enlisted in the Royal Irish Constabulary in 1844 and was sent to Mohil, County Leitrim. He married in 1854 in Cloone to Elizabeth Price whose father was George Price. He was sent to Belfast in 1864. From what I can gather much of Ireland in those days was a dangerous place and the RIC had their work cut out for them. There is a name attached to Forsythe family but where it comes from none of us know and that name is Robert Walter Elvin. The theory is that this name belonged to an Uncle somwhere. My belief is that William Forsythe's mother's family were the Elvins. From what I can gather there were Elvins in Urney but most of them were actually living right over the border in County Donegal. What would be the best Parishes to search for the Elvins? In other words, which Parish and townlands are right next to Castlederg. I am having trouble actually figuring that out. Are there any Elvins in that are today?