You must buy the John Harrington (ed.), Modern Irish Drama (Norton), which contains the plays marked with an asterisk and some very valuable documents and criticism.
There will be copies of these texts in the SRC and in many cases elsewhere in Warwick Library's holdings. The bookshop will also have the following editions. You may find you do have to buy a certain number of these texts but they on the whole are not too expensive and I've tried to suggest the cheapest decent versions. Amazon is a useful source (order a few together to save on postage costs).
Use the Penguin or OUP editions of the Edgeworths (OUP World's Classics Absentee), the Penguin editions of the Wilde texts and if you need a Yeats Selected Poetry, the recent Penguin edition is a good one. Make sure that your editions of Castle Rackrent and the Portrait of Dorian Gray have their original prefaces – these are essential parts of the texts.
Colin Smythe publish a Boucicault Selected Plays – the only people who do. These Boucicault plays are also in the Dolmen Boucicault in Warwick Library (WL).
Colin Smythe also publish the James Stephens Insurrection in Dublin; A. A. Farmar publish good cheap editions of Somerville and Ross’s novels, including The Real Charlotte; Vintage do a nice edition of Elizabeth Bowen’s The Last September; John Banville’s Birchwood is published by Picador.
All the Beckett plays are in the Complete Dramatic Works (Faber) which might be a good investment or is in WL. Faber also publish O’Casey’s The Plough and the Stars.