Will any red dot fit on top of iron sights on my ar15? (sorry if it doesn't belong here I was trying to find somewhere to post this). I was looking at the trijicon rmr type 2 and was wondering if that just gets set in on top of the fixed sights.

That optic says it comes with a mount to attach it on top of a acog scope. ETA: that was just the first one I looked at, I guess they aren't all like that. I'm not familiar with that optic, but it looks like it requires a mount to adapt it to a pic rail.
Then:
Do you have a carry handle or back up sights? Is your carry handle detachable?
The only way you mount a red Dot "on top of your iron sights" is with a special mount the attaches in your carry handle using that hole in the middle and then it has a picatinny or Weaver rail on top of the mount to attach whatever optic you want.
If you don't have a carry handle and you have some kind of back up sight mounted on your uppers built in rail, then you mount the optic in front of the rear sight and may need a riser so the dot or cross hair is the same height as your sights.

Are you talking about mounting an RMR to a carry handle?  If so, get a carry handle picatinny mount and attach the RMR to that.

Carry Handle Mount

You can just mount the RMR on it's own raised mount...it will co-witness with fixed or flip-up BUIS.

RMR Mount

yes I have a carry handle.

You have a few options;

1. Install a carry handle mount and use the trijicon low picatinny base mount for your RMR

2.  Take off your carry handle, buy a rear BUIS, and then find whatever version you like of a co-witnessing RMR mount.

American Defense, LaRue, Scalarworks, Trijicon, Midwest Industries, UTG (if your desperate)....all make a co-witnessing RMR mount.

There is a carry handle mount that places the pic rail in front of the handle so the red dot can co-witness with the carry handle sights.  I'll look for the link. Maybe someone else knows where to find it too.

link

ebay

I usually priase UTG, but if you are buying something like a RMR for durability/something proven: grab something better. American Defense is good to go.

https://ibb.co/TcH1Hwb

Sorry I am a bit noobish when it comes to this stuff. My question would be, I could pretty much by any red dot and then just would have to get the right mounting tools to put it on correct? That is a pic of my ar15 and my dog lol

Since you've got a removable carry handle and rail up top, you can buy any red dot with a standard AR-15 picatinny rail mount. Most people would also run a backup iron sight at the back end of the rail. If you don't want to spend a lot of money, Primary Arms or Holosun red dots are solid.

In addition to this you can use your removable handle as a BUIS by chopping it down (with a hacksaw) just in front of the bolt, by the rear sight. This is a common modification to get a quick, easy and reliable BUIS that has been used for many years originated with Special Ops guys.

Pic for OP
Attached File

Yep, just remove the carry handle, add a folding rear backup and mount your red dot on a riser mount ahead of it at either one of the common heights, either with the dot on the same sight plane, “absolute co-witness” or a bit higher, known as “lower 1/3 cowitness”.

My Deltapoint Pro is at absolute height.