Whilst we all agree it seems that time is the major component, it's worth further clarifying, if it wasn't already obvious, that you can tailor that time.
There's two general schools of finding. 1. Be a bird finder, but find whatever happens to crop up for you or on your patch. And 2. Cultivate a self found list, usually requiring you to put in effort to fill gaps in that list. This means that you can identify times and places you wish to be in order for you to have the best chance of finding those species.
For many species this can be very straightforward. As I've already said, if you haven't found a Buff-Breasted, Pectoral or Semi-palmated sandpiper, the first two weeks of September in Kerry will usually get the job done.
Need a Lapland bunting? Same time period on Mizen head, Carrahane strand, Cape Clear will fill that need etc.