I read something recently (possibly in Saturday's Irish Times) about how United are ramping up their presence in Ireland by teaming up with 5 travel agents to provide more access to OT. Wallace Travel was one.
You could also try which I find good. As the name suggests, tickets may only be swapped or sold at face value (or less).
Obviously I've no connection with any of the companies or the website.
You will just be fleeced by these "agents" who throw in a nights accommodation with a £40 match ticket and charge £250-£300 for the package without flights. If you know anyone in one of the supporters clubs they woul be your best bet for a ticket , obviously the games against Man C. , Liverpool , Arsenal , Chelsea are usually sold out but if you are prepared to go to see MU a mid-table team you should have no problem picking up tickets.