Well first off, the UK is VERY multicultural - we have just about every nationality you can think of and more besides living here!
Being half Irish or half black won't be a problem at all. Contrary to what bad history may have gone between our two countries (at the hands of the Government, not the people), the ordinary British people do not 'dislike' the Irish in any way whatsoever, in fact we like them a lot! Irish people and Irish culture are generally very popular in the UK and being half Irish will be a good thing when it comes to fitting in and making friends. Most UK cities have large Irish communties as well. We also have a huge black and mixed race population in the UK! Racism/standing out MIGHT really only be a problem in very remote,closed, rural community areas like Mid-Wales or the Scottish Highlands and you'd be unlikely to move to those places anyway as they're rural with few jobs. In any town/city you'd just blend right in!
The only problem you might have would be with being American - you will get some hostility for that, particularly from the anti-war lobby who want to blame you for everything George Dubya Bush has ever done, but thankfully these people are not the majority.
Getting a job and being successful in the UK depend on what you want to do. Sadly we have a large glut of arts graduates (including me - History Graduate!) who can't get jobs in their specialised area. The tide is turning more in favour of science, medicine etc. There is also a lot of competition from the EU for jobs over here.
The UK is also a very expensive place to live at the moment. House prices are unaffordable even for young graduates and wages are not great.
You also won't find it easy to get a work permit because of the new points-based system which has really tightened up non-EU immigration (you have to be highly skilled to emigrate to the UK from anywhere outside the EU). You might actually find it easier to come here on a student visa, maybe studying for your postgrad at a UK university or something.
Good luck! Hope you do decide to come to the UK. It has its bad points but I for one wouldn't live anywhere else!