Edward McLysaght in his book "The Surnames of Ireland", page 119, states that (Mac) GARTY is a variant of GERAGHTY. On further research I have found that GERAGHTY has about 14 variants and GARTY is one of them, mainly found in County Westmeath. As many people could not read nor write in the 1800's in Ireland they probably did not care how their name was spelt and it was left to someone like the local priest to make up the spelling. hence you get naming variants. May be CARTY is another one.
I have records from the RC parish of Tubberclaire for the baptism's of the children(Ann, Edward, Margaret, Patrick and Susan) of Patrick GARTY and Mary BYRNE between 1830 and 1841, so the name GARTY did exist in that parish.
All the best,