MartinTWD

As far as the 5 weeks goes, I would not consider that alone to be a big red flag. To me it seems the amount of time someone would travel for if they wanted to get their money's worth, you know? Even people with full-time year-round jobs might sometimes take a 5 week trip.
 
Ultimately, it is what it is. Try not to worry over it more than necessary. It seems like your past experiences have been overall positive (despite the trip to secondary) so that is a small indicator that your general personality does not cause most immigration officers to dislike you without cause.
 
I made multiple trips to the US in one year. The time I entered for three months the IO warned me that I might experience greater scrutiny on future trips. However, when I next returned (having been exceedingly cautious and stayed in the UK for an entire five weeks lol) I was let in with no trouble. (It's likely that I would have experienced more trouble on that last trip if we had not already been in the visa process, because it was a rather large proportion of time that I was spending in the US.)

You were not traveling too often nor staying a long time. I don't know why you got an interview the third time. 
 
Since you've been out of the US for 5 months between trips, you should be alright, at least comparing your case to other people on VWP. However, be careful about  what you say at POE because you don't have strong ties to Ireland if you are on disability. Maybe a K-1 would be best for you, or you girlfriend could visit you in Ireland and you can get married there. 
 
 
 

How many and how long were your trips before you get interviewed for 30 minutes? How long did you stay in March?
 
People who have said that they were getting married at POE had strong proof that they were going back to their country and filing a spousal visa. Without strong evidence of that they will send you packing. 

It is visa fraud to attempt to enter the US via a non immigrant visa to marry and remain.  You can enter, get married then return to your country while you wait for CR-1 processing.  You must always be completely honest when speaking to immigration authorities.  There is a high risk you will be denied entry unless you can prove very strong ties back to your country.

Telling them you might get married on this trip will probably get your entry denied immediately.