We should not accept Adams' use of language. The IRA has no authority to wage war. What we have had this past 30 years is not war, but unlawful and unjustifiable violence committed by an illegal organisation in pursuit of a political objective for which democratic methods were available.
Using Adams' reasoning, any person taken prisoner by the IRA is a prisoner of war and entitled to the same treatment as it demands for its own members. In practice the IRA murders its prisoners, usually dumping the bodies at the roadside. In the case of the "Disappeared", about 20 people were abducted, made prisoner, murdered, and their bodies concealed in secret graves.
To say that "these things happen in war" is to imply that they are not unusual, but are unfortunate, accidental consequences of military operations. In the cases of which Mr Adams was speaking the deaths were not accidental but planned, deliberate, premeditated-meditated acts of murder.