I will be leaving for my study abroad in Spain next week and was wondering if I will need a UK/Ireland visa for a possible weekend trip up there? I know that they are not part of the Schengen visa agreement but will they require a visa for a trip that might be under 48 hours?
Yes. Indian citizens need visa for entering either the UK or Ireland, and a Schengen visa doesn't change that; neither does the length of your planned stay.
The UK does issue a category of Visitor in Transit visas which are valid for up to 48 hours (and are presumably easier to get than a general visitor visa for their intended users). However, that is not applicable for you -- it's only for when you're passing through the UK on your way to somewhere else, and you'd have to provide an itinerary showing that with the application. And it would still count as "needing a visa"; it must be applied for in advance, and so forth.