It is not odd at all for both revenue and SW to carry out inspections together as there are Joint Investivation Units between both depts.
It may be a general inspection of the books (PRSI and Revenue issues) and they normally would tell you what they need so that you have the accounts to hand. It may also be acting on a report of abuse of the Revenue/Social welfare codes. It is generally a very detailed audit of the books from a tax/PRSi point of view