Holiday Visa for Vietnam 2019

I am travelling to Vietnam from Dublin Ireland on the 5th June 2019. It is our intention to visit Cambodia and if possible Laos while we are on holidays . In fact it may be a case that we start off the holidays in Hanoi Vietnan and travel at some point to Cambodia/Laos and then back to Vietnam . At this juncture we have not finalised our itenerary so I appreciate that this may be difficult to answer. My concern is that if we visit other countries e.g Cambodia etc and then return to Vietnam do I need a separate visa for going back in (re-entry back to Vietnam) Can I simply obtain a Visa for a three week period and will this afford us the flexability to travel for the entire period of the holiday. As an Irish citizen where do we submit papers