It should make no difference where you are when you book them (the timing of the booking might make a difference but that is very difficult to "fine tune" and likely the sweet spot would be when you are in Ireland) if you know your dates then I would suggest booking now, that way you have your details before you travel and can plan better. For example, baggage restrictions and allowances can vary from airline to airline and plane to plane and turning up with too much luggage for a flight can be very expensive. Use a service like
SkyScanner to work out your journey.
Internal flights are fairly straightforward on arrival, but courtesy of 9/11 they have become more like international flights in terms of departure. You have to go through the TSA palaver, gone are the days of walk up and walk on.