What are the best strategies for an Irish partner with a complex travel history returning to Thailand for marriage?

Similar details to OP, but some differences.

I’m American. Engaged to a Thai national. Marriage ceremony May 15 next year in Thailand. I understand the ceremony is only a ceremony and is not legally binding by itself, must go to US embassy, translator, and the Thai office to register the marriage and make it legal.

My travel history, all visa exempt entries followed by returning to America each time after departure:

June 14-27 of 2024

Feb 18-march 5 of 2025

Nov 5- 17 of 2025

April 8-20 of 2026

It seems I will stay a minimum of 30 days next entry, from last week of April through late may. I might try extending additional 30 days depending on how registry process goes.

Should I do my next entry as an exempt entry possibly followed by a 30 day extension? Or should I try to get a visa?

Use friendly Thai visa they can get him in.

It's not guaranteed but he will have a better chance on the METV than visa exempt. Have the 20,000 cash, proof of accommodation, and a ticket out. METV is multi-entry for six months too so that gives you some time to get the marriage sorted out (congratulations BTW). With the METV it would probably be lower risk to not plan on doing an out and straight back border run at the 90 day mark (after an extension). If you can, try to figure out the details and how long it is going to take and go on holiday to Vietnam for a month if you need to go and come back. If you can show you have that in process, that may also help in terms of getting back, I don't think it's anything like the US where showing intent to marry as a tourist is a (big) negative, I feel it's more likely to be a positive.

It will help as well if you are there, if he is questioned. No harm to have a brief letter describing your relationship, and copy of your Thai ID card and house registration, in case you are separated. And phone number so the IO can call you, they sometimes do want to do that, to confirm.

He will need open and put the money in the bank to do the conversion and there are stamp time remaining requirements that vary by office. You are on the home straight once married though and would always have the option of going to Laos to get the non-O, they will be quiet again now they have got rid of all the DTV applicants. Applying there he can use money in either home or Thai account and no seasoning requirement.

I am a USA citizen and I always follow my Thai wife thru Bangkok immigration. Even though is says Thai only. Never been hassled.

How does that works that you apply for a DTV and get a METV? Is like asking for a Soda and getting a fries.

If he has a METV VISA Stamp, there should be no issue entering Thailand 🇹🇭 and no questions would be asked by the Immigration Officer since he’s been approved and it’s a multiple entry stamp. And yes, go together with him so you can support him if there are questions.

Is he over 50 as a Non-O would get him entry?

I believe he would be able to walk with you in the Thai immigration line. From there it's up to the immigration officer weather he gets in Thailand or not.

You can try going through the Thai lane but it probably won't make much difference. Best advice I can give is for him to make sure he has accommodation booked, a ticket out of Thailand within the time he'll get stamped in for and a minimum of 20k baht or equivalent in cash.

Something else to be aware of is that it can take time to get married, and once you are many immigration offices won't process a marriage visa for a further 3 months

Have you considered applying at a consulate or online ahead of time?