Ryanair have a very decent selection of flights from Manchester to Dublin in September and October starting @ £9.99 one way.
Great for hen and stag parties or people just wishing to visit.
There are plenty of Thursday to Sunday flights with multiple flights per day to choose from.
eg leaving Thursday 11th Sept leaving 07:45 - 08:50 £9.99
returning Monday 15th Sept leaving at 06:20 or 21:35 £9.99.
Many more dates available at time of posting.