I'll answer that for you as I reckon I represent the average Irish man.
The answer is simply no. I've been to Celtic Park albeit not for an Old Firm game (I am a Cork City fan not a Celtic fan by the way). We do not carry on like that at International or club matches in Ireland. The tricolour flies in much the same way you'd see the saltire fly at Scottish football matches and is not seen by the different communities in the south of Ireland as a divisive emblem. Sectarian songs are not sung at LOI matches or at Lansdowne Rd (I've not been to the Brandywell in Derry where it may or may not occur), in fact there is no sectarianism in the south of Ireland despite the fact that Protestants make up a significant % of the population. Northern Ireland might as well be on the other side of the planet given the ignorance of issues in the north by people in the south (pretty similar to the ignorance of Londoners to Scotland in general).
i'd like to point out that I've been to an OF game at Ibrox. Having been invited by a Rangers supporting mate we jumped on a Rangers bus in Edinburgh and to say the journey to Ibrox was eventful would be an understatement. I watched the match from the club deck where various individuals including a very irate chap beside me hurled abuse at the Celtic fans for 120 minutes. Yip the match was a league cup semi-final which went into extra time. My point is for the 120 minutes I did not recognise anything that represented Britain never mind Scotland (I spent 7 years in Scotland and 3 so far in England).
Both Celtic and Rangers are strange enigmas. They both claim to represent something or somewhere yet appear to be at odds with those places. The only places that truly represent Scotland and Ireland in a footballing context are Hampden Park and Lansdowne Rd. You will not hear or see any bullshit in these stadia.
Onto your second point, there is no doubt about it that the Irish tricolour has been misused. It was created by Irish nationalists in the 18th century (many of the leaders were Protestants believe it or not). The idea was that the flag represented the people who lived on the island at the time i.e. green represented the Catholics, orange represented the Protestants and white represented peace between the two). Unfortunately historical events took over in the 20th Century and the flag is now seen primarily (at Celtic matches and in Northern Ireland) as a nationalist (catholic) emblem used to goad the very folk it partially represents. A sad state of affairs.
Any how enough of this bollix...I reckon there will be a minimum of 10,000 Irish in the stadium plus god only know's how many Scots supporting Ireland.
Top posting, mate. Far too sensible for this place! I am looking forward to sharing a few with the Irish support pre and post match.