My mother-in-law has dementia and forgets when we tell her something important.  How to deal with this without getting irritated at her?

Maybe join a support group for the caregivers of those with dementia. She's not going to change, in fact it's only going to get worse. So all you can do is learn ways of coping with the situation. 

Ask again when you have a serious problem.

My dad had strokes which caused dementia...

You answer any and everything like she is asking for the first time.Getting irritated isn't helping anyone.  The dementia isn't her fault so don't treat it as her questions are an annoyance even if they are.In this particular situation, it's a communication issue among family members.  If nephew took her to get a burger he could have informed someone.

My mom doesn't have dementia but she's had a long recovery from covid which caused some heart damage.  She's on a lot of meds, a few which make her forget things she asked or someone said, etc.   I'm with her the most but I often message my brothers things so they are aware, etc.   My brother comes and sits with her once a week for a few hours while I'm gone..  I will message him when she ate last, any meds she needs to take, etc.

With dementia, it's the short term memory that disappears first.  You seem to think your husband's call was "recent" enough that she should have remembered it, but it's actually the opposite.  She could probably tell you who her 4th grade teacher was, but she can't remember what TV show she watched an hour ago.  So yeah, it seems kind of counter-intuitive to ask her to remember something and then get annoyed when her memory fails her.  From now on, maybe just bring it by to surprise her.

You need to join a group of dementia survivors.  It's really hard to not become angry with a person who has this horrible condition and often people forget that the dementia sufferer honestly has no control whatsoever of what they can/can't remember.  However, that is the case, and it will only get worse as her condition progresses.  Be thankful that she's still able to get around and even talk to you.  There will come a time when she can do neither.  Think of her as a person who has suffered a massive head injury or stroke.  The effect is the same.

My thought is, you said you realize she forgets things but then you feel free to become irritated because someone has an illness which they have no control over. Who is childish here?

just save that lunch for another time

I mean, when you write it out like this and put it into words it does sounds like a petty thing to be upset over. It’s not her fault and I’m sure it’s not a position she ever wanted to be in. It can be frustrating sometimes but it’s good to remember her poor mental health might be an inconvenience for the people around her but it is what makes up her whole world (that was our family motto for my G aunt on a bad day).  

I always felt guilty when I would start to lose my patience with my great aunt Bonny because I knew it wasn’t her fault but I still would have to step away and take a breather sometimes so I could regroup and tell myself not to take everything personally. It can be a lot to deal with for long periods of time. 

If it’s something she would be comfortable with then it might help her to write stuff down in a place that she would see before leaving (like sticky notes at the door or a scheduled calendar hanging). - we had asked Bonny about this idea but she hated it. For whatever reason it agitated her.

Or talking to the rest of your family when you make plans. We had a group text for my grate aunt b/c she would mess her schedule up (She never handled the dementia well so we didn’t tell her about the texts. She would have hated it.) We would just text out a time and date if we were going over or taking her out so we would know the plans without getting any signals crossed. And that way when she started calling family members and making plans they would tell her that she already had plans or if she had an agitated day they would know who to call and talk to about canceling or changing plans. And it did good to keep everyone involved and coming around, it wasn’t a competition to see who cared the most but it did seem to keep everyone aware of their involvement or lack of without having to call anyone out or hound them to visit. 

It can be a hard thing to deal with, but I’m sure she appreciates the thought and the effort that your willing to put into visits for her! It took us some trial and error before we were really able to deal with it as a family (and a lot more communication then what we had previously - which was a big struggle for us at first) but after a while we figured out what worked for us. Keep at it, I’m sure your family will figure out a good system too. 

I hope when you get to her age, you have a daughter in law like you. You sound horrible. 

it's nothing to get irritated with, u nd to understand she has dementia. Try researching exactly what it is and ways to cope with it if a family member has it. you being very selfish and only thinking about yourself and reason why u feel this way. How about just calling your mother in law every day and saying hi, how are you? jux spend time w her and stop getting upset over petty things.