If you're planning on joining the Ironworkers union (or any of the other construction and/or welding related unions), no prerequisite welding experience is required. A written aptitude test is all they use to weed out the riff-raff. Call the closest Ironworker's local and ask if they're taking apprentice applications. If they are taking applications, ask to schedule a test date. Show up sober, presentable, and on time - take the written test. It's usually an all day affair but it doesn't cost you anything and there's no obligation whether you pass or fail. If you score OK and are offered an apprenticeship slot and decide to accept their offer, congrats your welding school is now paid for. (I just saved you at least 4 semester's worth of community college classes). Pay your initiation fee, pay your first month's dues, put your freshly printed member card in your wallet, and voila! You are now officially a union Ironworker. They'll tell you everything you need to do from here on. Go in from day one with a positive attitude and try to be completely teachable without any preconceived notions. In my somewhat educated opinion, going to a non-union welding school and then trying to join the Ironworker's is basically going to be a waste of your time and money and possibly set you up for failure.
Here is why I feel that way:
A welding cert isn't going to give you any advantage whatsoever on the written test. Either you know something or you don't - the union just wants to see if you have the basic foundations to potentially be a good Ironworker so a lot of the test involves really basic shit you probably learned in 5th grade like adding/subtracting fractions and simple geometry. All that a welding school diploma and a few certs would do for you as a prospective apprentice (assuming now that you did OK on the written tests and they're considering making you an offer) would be to possibly let you "test out" or skip over part of your apprenticeship training.
I totally aced my union's written exams (95% or better on the 8 different subject areas they tested on) so the union had me come back the next week to take practicals with their 4 year apprentices who were eligible for the Journeyman test. I aced two out of three of 'em and damn near passed the third practical. After seeing that, the union offered me a pretty sweet deal. Only from many years later and a lot of experience later can tell you in hindsight that it was a bad idea accepting that offer. Skipping early apprenticeship classes, even with full permission from the education coordinator, will get you at the least some minor distrust, and possibly even some undeserved animosity from other union members who consider it "cheating" to not start an apprenticeship from the very beginning. Starting my union career as a 7th period (80% Journeyman pay scale) apprentice gave me a big pay raise over the other noobs, but it also caused me a lot of grief because I missed out on some small lessons that other apprentices had learned years earlier. Some of them were making half what I was getting paid and we had very similar "real world" qualifications. See how that could cause some of the other apprentices to have a legitimate grudge?
If you do start as a first period apprentice (a "period" is about six months worth of on-the-job hours and classroom training, so a 1st period apprentice is basically a total noob), your union will teach you to weld completely from scratch exactly how they expect you to weld and that's actually an easier job if you're working with a clean slate. Journeymen who serve their full apprenticeships "the hard way" (no family member favors, no bribery, going the whole 4-5 year gig starting off as a lowly 1st period broom pusher) will generally carry a bit more clout on the jobsite as Journeyman and Foremen than anyone who did it "the easy way" (those who aced the Journeyman test and joined as a carded Journeyman, or did well on the test were offered to start out at a higher period apprenticeship and allowed to skip some lower period apprentice classes).
It's like any other business, you've gotta pay your dues if you want to go far. It takes time to earn respect and establish a solid reputation. That involves way more than just knowing how to weld. It takes a full 4 or 5 years to fully learn the skills (and perhaps more importantly, the politics) of the trade, so there's really no sense in wasting 1 or 2 of those years in welding classes that may, or may not, directly apply to what the Union actually wants you to learn.
As a first period apprentice, all you are expected to do on the job is keep your mouth shut and be able to follow safe directions. You'll learn more by being on the job than any classroom could possibly teach. Once you're a union member, you are getting paid to get an education, not the other way around. Keep that fact in mind when you're busting your ass for the first couple years.
Disclaimer: I am not a union Ironworker. I have worked directly side by side with them on the same jobsite and I feel confident saying our union "politics" and structure worked the same way. We both welded the same shit and went though lot of the same training (just under different union cards). Don't ask me to go into further details because I won't go there. I don't recruit for my former union or endorse them, but I withdrew my membership on honorable terms and I won't trash talk them either.