Herrera, Fletcher and Carrick are all better players than both McCarthy and Barry. Cue some Irish poster coming in and telling me I'm wrong, of course. The Everton pair are good players, but they'd face the same stick our lads do if they were here. Expectation is a massive influence in terms of how players are rated.
At City, Barry, despite his regular good form, was routinely hounded by rival supporters. As soon as he heads out on loan to Everton, plays exactly the way he was at City but for a team not as good, there are calls for his inclusion in the England squad and so forth. It's all very fickle.