you don't need the 'is', seo cofaidh, is simply this coffee, idiomatically it may be understood as this is coffee, but in other situations, you are making the statement, this coffee, as opposed to that coffee or any other coffee. If you want the 'is' in for absolute clarity, then either use a construction using 'tha' or a rarer 'is' construction. Not much used in every day speech now, but still there should you wish to make use of its different functions as opposed to 'tha'. Nach 'eil sin ceart? Tha. 'S eadh gu dearbh!