Much as I despise the whole Sund-Ireland concept and the idea that people go to England simply to watch any team that has any sort of Irish connection, you did ask for advice so:
Don't go anywhere near a travel agent, if you can't manage to organise a dirt cheap travel/ticket combo on you own for any of those teams you mentioned I'd be very surprised. Sunderland had 4,000 empty seats on Sunday I heard, so ticket still are not a problem. You won't get tickets for matches against the "big 4" for any of those clubs or a Sund-Newcastle derby, but other than that you should have no problems. Man City would be the ones most likely to sell out if they go on a run, but that remains to be seen.
Bear in mind the atmosphere at most matches in the Premiership is brutal compared to Cork City and if it's top-class football and a good atmosphere you're after head to the Bundesliga. Unbeliveable atmosphere and you can gargle in the stands/on the terraces.