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 on 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 Champions League defeat by AC Milan.
And the SPFL later confirmed it had begun initial enquiries into a similar display at Celtic Park days earlier.
Fans held aloft a large 'H' symbol - representing the Maze Prison block in Belfast 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".
SPFL chief executive Neil Doncaster said: "We are looking at that and as soon as we are in a position to comment further then we'll do so."
Celtic chief executive Peter Lawwell demanded supporters leave their political views at home after UEFA announced disciplinary action 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.
The fans' response read: "The club's statement claims that we assured them that our display would be purely football related but that in fact it had nothing to do with football.
"In reality however what was assured to them was that our plans were completely relative to Celtic and indeed football.
"Given that it is Celtic fans who are filling up prison cells and court rooms because of the Offensive Behaviour at Football Act, and the manner in which it criminalises legitimate expression of political opinion, this display could not have been more relevant in the current context.
"The fans are the lifeblood of any football club and to deem fan issues as irrelevant is frankly disgraceful."
UEFA rules prohibit messages of a political and ideological nature but the SPFL's regulations on "unacceptable conduct" are different.
Their description includes any display of "support for, or affiliation to, or celebration of, or opposition to an organisation or group proscribed in terms of the Terrorism Act 2000".
The home club has to ensure "as far as is reasonably practicable" that policies are implemented to prevent unacceptable conduct and that any such incidents are dealt with.
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