The only Tundergarth I know is two miles east of Lockerbie in the valley of the Water of Milk; it is certainly in an area where you might expect to find Murrays. This surname is often associated with Perthshire, but there was a separate landed Murray lineage between the Annan and the Solway south-east of Dumfries who had principal houses to the west of Ruthwell at Cockpool (now a hotel) and at Old Hoddom (a ruined 19th century folly of a castle in a caravan park) about six miles south of Lockerbie. Don't jump immediately though at landed ancestry. There seem to be a mix of Murray origins here, some traditionally Highland Murrays who stopped off here on the retreat before Culloden, as a convenient place to lose themselves amongst other Murrays. Besides which Ecclefechan, about 4 miles south of Tundergarth, and Lockerbie provided employment at various times to migrant communities. You will therefore likely find a good few Murrays in Ecclefechan and Lockerbie in the 19th century, who might have lived further afield in the 18th century. There is quite a likelihood of of farm workers seeking work at farms in the area or as small tenants, so that could be a reason for finding Murrays at Tundergarth. The spot with strong Murray tradition here is Repentance Tower, a very small tower house on a hill half a mile south of Hoddom Castle. Basically you will find a lot of Murrays along this stretch of the Annan Valley, and a lot of history associated with the name. But you can find Murrays anywhere across here from Dumfries to Eskdale and from the Solway to Moffat.
