Calling Lennon a numpty could never be interpretated as being rascist or bigotted, nor could it be interpretated as showing a degree of abuse towards the player so there's probably little risk of being prosecuted. Call him anything that may refer to his race, colour or creed and thats a whole different ball game, and one that I would not defend. And nowadays, even referring to our neighbours across the bridge as gypo's, because the Gypsy Traveller community has won the recognition of being a race of people, could get people into serious trouble with the law no matter how innocent or funny it may seem to the individual. The whole idea is to try and stamp out the rascism and bigotry that comes from the terracings.
If the song you refer to is 'Fields of Athenry' then it has been challenged and found not have anything to do with bigotry. The words don't display hatred or malice towards anyone of another race, colour or creed but is about the deportation of wrongdoers, that happened all over Britian and Ireland, at the end of the 19th century. Indeed the song has been adopted by teams in the English leagues, as well as the tune being used by Rangers fans, and is becoming as much a football song as 'Never Walk Alone'.
I do agree though that songs of hatred need to be stamped out but how do you do it. If 2500 OF fans, at ICT stadium, start singing offensive songs, how should the police handle this. The risk of hostility needs to be assessed to ensure the safety of the innocent. I have no doubt that the songs of hate are mentioned in the match commanders reports, which are submitted to the relevant authorities, but like anything else it takes time for actions to come about.