Stuart Gilhooly, a lawyer from the Professional Footballers' Association of Ireland, also said the rape Evans was convicted of was "at the bottom end" of sexual assault and "alleged" - despite the fact the conviction stands.
"The argument against that is that a jury convicted him of the crime. The same applied to the Guildford Four and the Birmingham Six. They got no public sympathy either," Mr Gilhooly wrote.
"This crime, as alleged, was at the bottom end. There was no violence and thankfully the victim has no recollection of it. This, I hasten to add, does not make it right, or anything close to it, but it is nonetheless a mitigating factor.
"It's not easy to muster up too much sympathy for Evans but there is surely nothing worse than being accused of a crime which you genuinely believe you didn't commit."
The controversial remarks came after former Housemartins and Beautiful South frontman Paul Heaton stepped down as a patron of Sheffield United's Community Foundation, following the examples of TV presenter Charlie Webster, 60s pop star Dave Berry and Sheffield businesswoman Lindsay Graham since Evans returned to training with the club.
Wales international Evans was jailed in April 2012 for raping a 19-year-old woman in a hotel room in Rhyl, north Wales.
The striker denied raping the woman, saying the sex was consensual, but he was found guilty by a jury at Caernarfon Crown Court.
An appeal against his conviction was rejected by three judges at the Court of Appeal in 2012. His case is due to be looked at by the Criminal Cases Review Commission.
The Guildford Four were imprisoned for 15 years for a bombing in Guildford carried out by the IRA.
They were released after it was revealed much of the evidence against them had been fabricated by police.
In 2005, then-Prime Minister Tony Blair said he was "very sorry that they were subject to such an ordeal and injustice", adding they deserved "to be completely and publicly exonerated".
Reaction to the Ched Evans comparison was severe, with people claiming they were "gobsmacked" at the claim and calling the Irish PFA "muppets" for suggesting it.
So, the Irish PFA have published article on their website comparing Ched Evans to Guildford Four. Its by PFA solicitor. #Gobsmacked— Anna Kessel (@Anna_Kessel) November 17, 2014
Are Guilty/Alleged/Innocent synonyms in Ireland? Irish PFA’s lawyer compares Ched Evans’ treatment to Guildford Four http://t.co/LuexLhhEtE— Peter Savage (@pcsavage) November 18, 2014
Oh - it was only an "alleged" rape. Someone call the judge and jury and tell them it was just a dream. FFS! http://t.co/1U4qUMx1nv— Jean Hatchet (@JeanHatchet) November 18, 2014
Simple looking man from Irish PFA compares Ched Evans to the Guildford 4, and writes worst thing about anything ever:http://t.co/lMdCbezmWU— Not Chris Taylor (@the_itch1980) November 18, 2014
Stuart Gilhooly is the solicitor to the PFA Ireland. Presumably this is his resignation letter.November 17, 2014
The Guildford Four served 15 years for something they didn't do. The Irish PFA has compared them to Ched Evans. http://t.co/O7NhiCMy0B— Daniel White (@danwhitepr) November 17, 2014
Mr Gilhooly went on to say Evans could not apologise for the rape so long as he was maintaining his innocence.
He wrote: "How can Evans apologise? He is convinced, and has a good argument, that he is not guilty of rape. If he apologised, then he is admitting a crime he feels he didn't commit. Who would do that?
"But Ched Evans has served his time, whether he is innocent or guilty. He puts the ball in the net so eventually someone will take a chance on him."
Campaigners against sexual violence have criticised the club's decision to allow Evans, 25, to train with the team.
Olympic athlete Jessica Ennis-Hill asked for her name to removed from a stand at the club's Bramall Lane ground if Evans returns full-time.