In 1852, Lord Ventry owned the land and John O'Donnell was his only listed tenant.
In turn Maurice O'Donnell, leased land from John as was common practise.
http://www.askaboutireland.ie/griffith-valuation/index.xml?action=doNameSearch&PlaceID=1679777&county=Kerry&barony=Corkaguiny&parish=Kilgobban&townland=%3Cb%3EGlannagalt%3C/b%3E
39 baptisms and 5 marriages are recorded here for O'Donnell Glenagalt (notice how the spelling has changed!)
http://churchrecords.irishgenealogy.ie/churchrecords/search.jsp?name2fm=&name2l=&namefm=&namel=O+Donnell&location=Glenagalt&dd=&mm=&yy=&submit=Search&sort=&pageSize=100&diocese=KERRY+%28RC%29&parish=¢ury=&decade=&exact=&ddB=&ddM=&ddD=&mmB=&mmM=&mmD=&yyB=&yyM=&yyD=&locationB=&locationM=&locationD=&member0=&member1=&member2=&member3=&member4=&member5=&member6=&member7=&member8=&member9=&namef0=&namef1=&namef2=&namef3=&namef4=&namef5=&namef6=&namef7=&namef8=&namef9=&namel0=&namel1=&namel2=&namel3=&namel4=&namel5=&namel6=&namel7=&namel8=&namel9=&keyword=
In 1901, there are only 4 O'Donnells remaining in Glannagalt.
http://www.census.nationalarchives.ie/search/results.jsp?census_year=1901&surname=O%27Donnell&firstname=&county=Kerry&townland=Glannagalt&ded=&age=&sex=&search=Search&relationToHead=&religion=&education=&occupation=&marriageStatus=&birthplace=&language=&deafdumb=&marriageYears=&childrenBorn=&childrenLiving=
In 1911, there are none. But there are families named Murphy, O'Connor, Curran, and Foley.
Gleann na nGealt was so named because it was believed that water from the wells there was a cure for madness. This wonderful documentary explores the reality of this belief. The contributors all speak beautiful Irish but there are English subtitles.
http://nasc.tg4.tv/1422Eto