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The Rathvilly baptismal records start in October 1797 and the marriages in 1800 http://registers.nli.ie/parishes/0727 so likely any children born before Thomas preceded October 1797 so a record would not be available. The marriages start too late also. I would still check the first pages of the baptismal register to see if you see any children.
There was a Dennis Lawler in the 1826 Tithes for Rathvilly.
The 1852 Griffiths Valuation head of household listing for Rathvilly parish shows a number of Lalors including a Denis in Patrickswell which was one of the townlands in the Tithes. Might be the same Dennis/Denis but he would have been over 70. Maybe a nephew? http://www.failteromhat.com/griffiths/carlow/rathvilly.htm