Anyone know how these are attached? Am looking to possibly remove irons on a smoothbore deer barrel. Thanks all.

Factory sights are brazed on.
Removal is a major PITA.

You have two choices:
One is to heat the barrel to a red heat until the brazing melts and the sight can be removed.
This ruins the barrels finish, and can damage the bore.
After the sights are off, you'll have to grind and file to remove the last of the brazing, re-polish the bore, and refinish the barrel.

Second choice is to cut the sights off.  I used a jewelers "wire" saw to cut as much as possible off, then ground and filed to remove the rest.  The barrel will need a refinish.

Last and best choice: leave the barrel alone, and buy a new barrel.
You can keep the old barrel, or sell it at a good price on the gun forums or Ebay to fund the new barrel.
If all you want is a barrel with no rifle sights, new and used smooth barrels are fairly cheap, running from a replacement barrel made by Mossberg, up to new factory barrels.

Still another choice is to buy an older used 870 barrel without the rib and just cut it down.
Many pawn shops, gun stores, gun shows, Ebay, and the gun forums have used barrels.

ABSOLUTE LAST choice is to remove the rifle sights.  

OK thanks. My girlfriend owns this gun and wanted the sights off. Not my choice, but figured if I could do it easily I would for her. Thanks for the info, seems the sights are much better off staying on the gun.