A gala opening will be held on 25 July following a Â£12 million renovation project, with Bellowhead the main act.
Cardigan Castle is believed to have been home to the first ever recorded Eisteddfod in 1176.
Some locals believe it is a "gross faux pas" to enlist an English group to play at such an historical cultural event.
The renovated castle- which has new accommodation, a cultural centre and restaurant- hopes to attract over 30,000 visitors in its first year.
Organisers the Cadwgan Trust have enlisted Welsh group 9Bach to support Bellowhead, who it describes as 'one of the best live acts in the UK' for the near sell-out 800 crowd.
A pressure group was set up to protest. It is called The Friends of Rhys ap Gruffudd - who was the ruler of the old kingdom of Deheubarth in 1176 when the first Eisteddfod was held.
Spokesman Hefin Wyn urged the Cadwgan Trust to rethink, saying that an English act headlining an event of such historical and cultural significance was a "gross faux pas".
He said: "Perhaps they are suffering from cultural and historical amnesia. They haven't grasped how important the occasion is."
Local Councillor John Adams Lewis told BBC Wales that "there is tremendous talent in the locality and some of those should be taking part."
In a statement, Sue Lewis from the Cadwgan Trust said that there would be a "superb programme of events" at Cardigan Castle over the summer that would feature "national performances" and "up-and-coming local talent."
A row has erupted in Cardigan over plans to invite an English folk group to headline the opening concert at the historic town castle.