Fly-half O'Gara, Ireland's most capped player and scorer of a record 1,083 points for his country, has struggled for form and was relegated to third-choice number 10 for last month's match against Scotland which Ireland lost 12-8.
Sexton, who tore a hamstring during the 12-6 defeat by England on Feb. 10, Paddy Jackson, who made his debut against Scotland, and uncapped Ian Madigan were the three flyhalves named in the squad by the Irish RFU on Sunday.
Locks Mike McCarthy and Donnacha Ryan and wing Craig Gilroy were named in the squad despite nursing injuries.
Ireland are fourth in the Six Nations table with one win from three matches.
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