Should one include irrelevant background on CV after career change? If one changes careers/subjects dramatically,  would it be necessary in the interest of full disclosure to include irrelevant aspects of the educational background and work-experience etc. ? 

For instance if somebody has an undergraduate degree in politics and 2 years of experience in PR but then goes on to an academic career in theoretical CS (essentially starting from scratch i.e. including an undergraduate degree), would it be better/necessary to clutter the CV by mentioning that earlier background ?

I guess there must be a certain degree of flexibility, as people would not include say a part-time job in the cafeteria etc. pp. but I am not sure what the academic conventions are in different scenarios. ( I am thinking about this in the context of applications for conferences, funding, workshops, travel funds etc. pp. )
The context for what the CV is going to be used for is key. I have a single CV that includes "everything" since I started grad school and selected things from before then including seemingly unimportant part time jobs. Having a long CV makes it easier for me to create short CVs because it means I need to delete things instead of remember things.

For your intended context, I would not include the work experience, but might include the degree in politics. Including that degree will likely lead people looking at your CV to think you are older (which might be a pro or a con). If you are worried about age discrimination, I would drop it completely.