I would strongly recommend a visit to Charles Byrne's music shop on Lower Stephen's Street. It is extremely suited to a guide such as this, since it is not a mainstream shop but offers specialist expert advice on traditional and classical musical instruments. The staff are very friendly and this shop is well-known by Irish musicians. It's also very near the Civic museum, the George's Street markets and Dandelion Bookshop so do call in and see what's on offer. They also offer an international mail order service through their website listed above.
Around the corner is Dublin's best classical music shop, Opus 2 on South Great George's Street. You'll always get good service and stock and you can order music by e mail, phone or post. Almost across the road is Walton Music, experts in Irish traditional music as well as a good stock of sheet music. A short walk northwards to Cecilia Street in Temple Bar leads to Claddagh Records, an excellent source of Irish recordings with tapes from 4 Euro. McCullough Pigott, phone 00-353-1-677 3138, 670 6702 is Dublin's oldest music shop (although they have moved twice in recent years) on 11 South Williams Street, Dublin 2 (parallel and to the west of Grafton Street). They have a host of friendly staff awaiting your enquiry about Irish music.
A visit to the birthplace of Thomas Moore, only a few minutes walk from Charles Byrne's shop, is warmly recommended since it is now a pub! You'll find it at 12 Aungier Street at the junction with York Street. Moore wrote such famous lyrics as The last rose of summer and The meeting of the waters. There is a small plaque commemorating the place but little else - make a hallowed pilgrimage to 'J.J.'s' pub if you're a Moore fan! Aungier Street is a continuation of (Gt.) Georges street and Wexford st.
Dublin also boasts the headquarters of Irish Traditional Music, Comhaltas, is located in Monkstown in the suburbs of Dublin (on the DART). Their website, www.comhaltas.com, is full of useful information and links. They frequently organise tours all over the world. One very useful resource is the web version of Irish Music Magazine. This is an Irish-based Magazine with excellent links, along with indexed back issues of the Magazine.