First found in Cork in ancient times, it comes from the gaelic
"O Tuama".
http://www.houseofnames.com/xq/asp.fc/qx/twomey-family-crest.htm
Toomey, O'Toomey and Twomey are Anglicized versions of the Gaelic O'Tuama (descendant of Tuama) with Tuama being a personal name derived from "tuaim" 'which meant
>>>>>>>>>"small hill."<<<<<<<<<<<
http://www.4crests.com/surmeantz.html
Twomey Genealogy Message Board
http://genforum.genealogy.com/twomey/
Twomey at Go Ireland
"The most distinguished man of the name, Sean O'Tuama (1706-1775), was born and lived all his life at Croom, in the neighbouring county of Limerick; there he and his wife kept a publichouse which was the meeting place of the Maigue Gaelic poets, he himself - "O'Twomey the gay" - being the most distinguished of them."
http://www.goireland.com/genealogy/scripts/Family.asp?FamilyID=310
Family Tree DNA
http://www.familytreedna.com/surname_join.asp?code=W10176&special=True&projecttype=G