This was clearly a political statement and not a racist attack. If anything, he's calling himself a 'nigger', and not as a way of smearing black people but as a way of comparing the way blacks and Irish have been considered historically. The comparison isn't about what happened to each group, but of the denigrative attitude the prevailing powers had of them. It might not have been the most graceful of tweets, but in general I'd defend his right to say it.

On a side note, I wish people would be as concerned about the racism the word represents as they are about the word itself. Going nuts over any use of the word has become a kind of replacement for actual anti-discrimination progress. You see this disconnect in America in particular, where powerful people can lose their jobs for using the word 'nigger', but at the same time you can have black communities being terrorized by the police, discriminated against by law, in housing and in education, and have blatantly racist policies such as the repeal of the voting rights act passed. But if everyone makes a big noise about saying nigger, you can make it look like you're a progressive country.