Celtic could face action over a second banner after the Scottish Professional Football League confirmed it was looking into complaints about a display during Aberdeen's visit to Parkhead last Saturday.
Uefa opened disciplinary proceedings against Celtic on Wednesday over an "illicit banner", including images of IRA hunger striker Bobby Sands and Scottish nationalist icon William Wallace, which was displayed ahead of their 3-0 Champions League defeat by AC Milan.
And the SPFL later confirmed it had begun enquiries into a similar display days earlier.
Fans held aloft a large 'H' symbol – representing the Maze Prison block where 10 Irish Republicans died on hunger strike in 1981 – alongside lyrics derived from Flower of Scotland, "they fought and died for their wee bit hill and glen".
"We are looking at that," said SPFL chief executive Neil Doncaster, adding that further comment would be made later.
Celtic chief executive Peter Lawwell demanded supporters leave their political views at home while the club vowed to ban anyone involved in displays which "were in no way football-related".
But the supporters group responsible for the banners, the Green Brigade, later defended their displays and insisted they were football-related as they were designed to protest against the Scottish Government's Offensive Behaviour at Football Act and Police Scotland's implementation of the legislation, which they claim has criminalised expressions of Irish politics.
A statement said: "To deem fan issues as irrelevant is frankly disgraceful."