You need to take a fairly relaxed approach to spelling in Ireland. There was no consistency and each spelling was down to the whim of the official recording the information. So Drumlister and Drumlester are the same place. Drumlester is the more usual spelling.
Sixmilecross (spell it as you like) is the electoral district that the townland of Drumlester is in. It is also the name of the nearby village.
A townland is the smallest administrative area of land in Ireland. They can vary in size from 1 acre up to 5000 acres, though most are between 50 and 500 acres. The whole country is divided into townlands. Many are rural, and there is not necessarily any town in a townland. Indeed some have no-one at all living in them eg mountain tops and uninhabited islands. Originating in the older Gaelic dispensation, and dating back to the 11th century, if not earlier, townlands were used as the basis of leases in the estate system, and subsequently to assess valuations and tithes in the eighteenth and nineteenth centuries.
In rural areas there were no street names or house numbers (that is still the case in some remoter parts) and your townland was sufficient to identify you or get a letter delivered. The postman, and anyone else who mattered, knew exactly where in the townland everyone lived. They survive as important markers of local identity. (A townland is not the same as a US township).
Drumlester is 719 acres of agricultural land. The modern Drumlester Rd runs through it. Your Managh farm is listed in Griffiths Valuation for 1860 as being plot 6, which was shared by Charles & Alexander Managh. Each had a farmhouse, outbuildings and a half share each of 22 acres of land. Alexander also had a one third share of another 22 acres of land on plot 4. The main farm(s) on plot 6 were on the modern Drumlester Rd, just to the west of the junction with Cleggan Rd. Easy enough to find today should you wish to do so.
You won?t find the townlands and their boundaries on the average on-line map. Your best bet is to use the maps attached to Griffiths Valuation which have both the contemporaneous 1860s map and a modern map which you can overlay (using the slider bar in the top right hand corner).
Drumlester is in the parish of Termonmaguirk. They probably attended church in Sixmilecross. PRONI in Belfast has copies of the older records as follows:
C.I. Cooley or Sixmilecross (Armagh diocese) Baptisms, 1836-71; marriages, 1836-46; burials, 1837- 71; confirmations, 1837, 1840, 1843, 1846, 1866, 1870 and 1873.
It is free to access those records at PRONI. Later records are still held by the Church. (There may be a fee to pay if you wish to access the later records at the church/rectory).