Terrorists operate in different socio political environments and have different goals.
In Northern Ireland, causing civilian casualties amongst ordinary and non viciously partisan people could cost support from ones own side. Likewise, attacks on innocents could lead to retaliation from militants on the other side.
As a result, a certain number of IRA bombing attacks came with warnings. A good number of others, however, had no warnings. Whether or not a warning was made could depend on many things including the target, what Protestant militants had done or had not done recently, or the mindset of the particular IRA cell etc. etc. When warnings were given, they could be very brief.
Though casualties were minimized, the warned attacks were designed to produce a sense of panic (especially with multiple bombs in an area) and give the impression that the IRA could strike at will. Naturally, warned attacks came with credit claims and probably statements to the effect of: "Better listen to us, next time there might not be a warning(s)".