There is a distinction between an Irish flute and the kind you see featured in the symphony orchestras. Materials, for one thing. The orchestral flute has a metal body and matching keys, while the typical Irish flute, resembling more closely a woodwind in materials, is made of wood with metal keys, if any.
The present-day Irish flute, a simple six-hole model, is capable of playing most traditional Irish melodies in the keys of D and G, without challenging cross-fingerings. The Irish flute can have no keys, or it can be enhanced by the addition of metal keys to accomplish several objectives; e.g., improving playability, expanding the flute's range, and enabling the playing of certain pitches without cross-fingerings. With the addition of only four keys, a flutemaker can create a fully chromatic instrument.
Irish flutes are sometimes referred to as "Nicholson-type" flutes. That's because of London flute virtuoso Charles Nicholson (1795-1837), who had the most profound influence on the course of wooden flute playing. Nicholson's powerful playing on his modified flute inspired Theobald Böhm to develop the so-called modern flute. After the worldwide demand for Böhm's flutes made old wooden flutes virtually worthless, the Irish began buying them up and making a place for them in the Irish traditional music scene. Without Nicholson's inadvertent role in making wooden flutes cheap enough for anyone to buy, the Irish flute might have stayed high-priced forever, and never have become an esteemed traditional music instrument. (For related information, see the Irish Flutes History article.)
Flutes currently made for Celtic and folk music resemble the wooden flutes Nicholson popularized in the 19th century, with large holes; one difference is that the modern model features few if any keys. Some say that the Irish flute brings together some of the characteristics of a 19th-century classical flute with those of a baroque flute. The rest of us just like the way it sounds.