Kerry,
I had a look in the 1901 Irish census. There were 100 people in Ireland named Rennick. There were 18 in Fermanagh. 1 family of 6 were RC, the remainder Church of Ireland (Episcopalian). Also one Renwick family, which is obviously the same name. Again Church of Ireland. You could try and see if you can locate any of these families descendants today.
http://www.census.nationalarchives.ie/search/
I looked in the current Northern Ireland phone book. There are just 4 Rennicks listed. None in Fermanagh. However a high percentage of the population is ex-directory these days, or don’t have a landline, and so there will be more, some of whom may live in Fermanagh. The electoral register suggests there’s about 80 (over 18 years of age) registered in Northern Ireland but you have to pay to get the details.
http://www.ukphonebook.com/
The name looks to be English or Scottish in origins. There’s about 3000 of them in the 1901 English census, and a significant number in the Scottish census too. So your ancestors are very likely descendants of plantation settlers who came to Fermanagh in the 1600s. (Their denomination would tend to support that supposition too).
Many of the settlers in Fermanagh came from the Scottish Borders, and the name Rennick is found there to this day. So if you are going to go for DNA testing, whilst you might hopefully get matches in Fermanagh, you may also get matches in Scotland and northern England, because that’s where the family almost certainly originated.
Large numbers of troublesome Scots and English from the north of England were shipped over to Fermanagh in the 1600s from the Scottish/English Border area because of their long established and mildly irritating habit of stealing cattle and robbing travellers. Shipping them to Ireland killed 2 birds with one stone. It removed them from the lawless Border areas and also put them where new settlers were required in Ireland. They were known as Border Reivers (reiver = robber). If you Google “Border Reiver” you can read up about them. Romanticised by Sir Walter Scott, the Reivers were actually a load of trouble. A wild lot. Still celebrated today though. Hawick in Scotland holds an annual Reivers' festival as does the Schomberg Society in Kilkeel, Northern Ireland (the two often co-operate). Perhaps that’s your family’s background?
