It is in response to the rising customer demand.
The airline will now operate up to five daily flights on weekdays.
Since services on the route started in March 2022, seat capacity has increased in line demand from business customers in particular, who require choice and flexibility of flight timings.
Emerald Airlines will operate the ATR72-60 turboprop aircraft to service the flights between Birmingham and both Belfast City and Dublin.
Tickets are on sale now via aerlingus.com with fares starting at £34.99 one way.
Ciaran Smith, head of commercial at Emerald Airlines said: “Having received positive feedback since starting our Birmingham-Belfast City operations, we are pleased to be providing customers with even more options when travelling between the two cities. We foresee these additional flight times to be particularly popular amongst those travelling for business purposes, and we look forward to offering that increased customer convenience.
"The investment of additional capacity on the route further confirms the economic significance of the Birmingham region, and in particular their links to Northern Ireland.”
Tom Screen, aviation director for Birmingham Airport, said: “We are thrilled the BHX-Belfast City route is providing so popular. Aer Lingus Regional services deliver exactly what their customers want – a formula which has served them well to date and will continue to long into the future.”