Martin McGuinness, whatever his past, is the Deputy first minister of N. Ireland. He's also an MP, many of his constituents if not most being practising Catholics.
I think he has every right to make a statement on this matter whether you agree with that statement or not. If every politician who had a dubious past was prevented from speaking out there wouldn't be too many saying anything (who said that was a good idea - be quiet!)
Unfortunately not all countries in the world had the relative stability brought by government by concensus which has been enjoyed, for example, by people in other parts of the United Kingdom in recent times. That concensus now largely exists and it entitles all who contributed to bringing it about to enjoy the full rights that politicians etc. in other parts of the British Isles enjoy, including the right to act like normal politicians.