Dharmapuri is a district in Tamil Nadu. It was part of Vijayanagara & Mysore Kingdom, later it went to Madras presidency & TN. Though Tamil is major language now in Dharmapuri district, it was also historically part of Kannada region, as indicated by village names & inscriptions.
The mountains of Eastern Ghats can be termed the historical geographical boundary between Tamil & Kannada regions.The regions around this border perhaps were bilingual. Dharmapuri,Krishnagiri,Talavadi are part of traditional Mysore Plateau unlike plains of other Tamil districts. 
George Grierson in his well known Linguistic survey of India says , Kannada was widely spoken in the districts of Coimbatore, Salem and Nilgiris. Dharmapuri and Krishnagiri were part of Salem districts then.
There are more than 250 Kannada inscriptions found in Tamil Nadu. Out of which nearly 140 Kannada inscriptions are in Dharmapuri+Krishnagiri districts alone. Most older inscriptions of these districts are Kannada.
Even today Kannada & Telugu people form major population in Hosur & Denkanikote taluks & Kannada in Talavadi taluk & significant minority Kannada population in Coimbatore, Mettupalayam, Nilagiri, Sathyamangalam. 12.86 lakh people stated Kannada as mother tongue in census in TN.
Notice the so many Kannada villages names in Dharmapuri taluk ending with Halli ಹಳ್ಳಿ like Somenahalli, Budanahalli, Noolahalli etc.
