accessing merge screen through URL
Another way of accessing Merge is from any Search function (based on Contacts eg Find, Adv Search, Quick Search). Eg click in QuickSearch, hit Enter (gives you all Contacts), select any 2 Individuals, in Actions chose Merge, this gives you a url like this (but without the cool arrows Chris had)

civicrm/contact/merge?reset=1&amp;cid=2&amp;oid=3


As a further answer, if you have the paired IDs in an external spreadsheet, then use the CONCATENATE function to join together all the bits as a hyperlink, then you should be able to just click through to the merge url for each pair.