Tourist visa procedure

If you are only going for three days it seems that you can get a 15day exemption when you arrive in Hanoi. There is no need to apply before you arrive

No, no. OP says they are all Irish. The 15 day visa waiver program does NOT apply to the Irish, UK, French, Spanish, German, and Italians YES, Irish NO.

Oops, I forgot to answer the OP's question.
They can apply online directly to Vietnamese immigration for a 30 day evisa. https://evisa.immigration.gov.vn without paying an online processing fee. Ireland IS on the list of 40 countries that are allowed this option.

hi guys, im not up to date on what reply #1 says, it may be true i dont know. you can either apply and fill out an online application for a visa and then have the print out confirmation with you when you arive in Hanoi or if have you not left Ireland for your trip yet and have a few weeks before departure you can apply through the vietnam consulate, though i think your application may go to London and can take time. so online is probably your best bet.

No, no again. Well, you CAN do as ger_power suggests but it will take a long time AND cost double or triple the cost of an online visa.
Applying online at the link in #3 above cost $25USD for the stamping fee, no online service fee.
OP is only staying 3 days so I'm guessing the cheapest option would be best.
Go here for ALL the visa information https://en.wikipedia.org/wiki/Visa_policy_of_Vietnam
UNLESS your nationality is on the list that gets a Visa Waiver of some kind you can NOT just turn up and get a visa at the airport or land crossings. You either need a visa in your passport, an evisa, or a pre-approved visa letter to even get on a plane going to Vietnam

Thanks everyone for your help, especially cmburns. I've taken your advice and applied through that site, and seem to have succeeded! I'll know for sure in a few days.

Thank you!

Your welcome, it's always nice when people take the time to come back and say thanks.
I haven't used the Immigration site yet because I often need a longer or multi-entry visa. So, let us know how it all works out for you.

Will do! Won't be back till early Dec, so will post then.

Excuse me for jumping in, but this seems relevant to me also. I'm wanting to go to Vietnam the end of this month, traveling from Cambodia and hopefully crossing to Ha Tien. I've just been to the embassy here in Phnom Penh and was told I do not need a preapproved letter to gain entry. For $40 I can get the visa there. This contradicts everything I have read so far. (Thai embassy deja vu...). Can someone please confirm? I have a British passport and would like a one month single entry visa.