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The RC records for Kilteevoge parish start in 1855 http://registers.nli.ie/parishes/1009
The 1857 Griffiths Valuation head of household listing for Kilteevoge civil parish shows many McGlynn records which shows that the surname was very common in that area. Nine of the records are for Letterbrick.
The 1901 census for Letterbrick shows 10 McGlynn records.
I looked at the online Irish phone book (which is only a partial list of phone users) http://www.eirphonebook.ie/# and there were 55 McGlynn records (make sure you use the Residential Tab) None of the records show Letterbrick but there are some near Cloghan. You may want to write to these people.
Let me know if you have any questions.