You misunderstand the way in which the word 'Terrorism' is used in this context. 'Terrorism' is not a blanket term meaning any act of terror against a state, but a specific word meaning an act of terror aimed against the interests of the United States (or of its allies, should it be prudent for the US to think so).
The US is, of course, the single largest supporter of terrorism in the world, both directly through arms sales to places such as Indonesia (genocidal policies against East Timor (UK also very guilty of supporting this), Israel (far too many vetos of UN resolutions aimed at bring about a peace settlement, massive arms supplies, brokering of arms deals to outlawed states (i.e. South Africa under apartide recieved US arms via Israel), Nicaragua (support of the contras) and also has directly carried out military strikes against civillian targets such as mosques etc in Saudi Arabia, deliberate targetting of civilian targets in Vietnam, Laos, Cambodia etc etc (the list is depressingly long).
In this light, since the IRA poses no threat to any US interests, and indeed since there are many more Irish in the US than there are in Ireland, and it is politically expedient to do so, it is regrettably inevitable that Bush wouldn't count this as terrorism.
Moreover, the IRA has been virtually inactive during Bush's presidency. Additionally, it should be noted that the US policy of supporting terrorism is not one started by Bush by any stretch of the imagination. Indeed, it is merely US policy.