Are you flying through london? Any chance they could meet you there for a day or two if you're feeling really brave so that it's only putting you out for a few hours rather than a whole week?
What does your dh think? Does he really want you to go to his parents too or is he not that bothered? And if he does want you to go - would he be perfectly happy to go to stay with your parents in similar circumstances (with how your relationship is with your mil and how she will treat you rather than how well or not he gets on with your mum!).
I'd just say that I really didn't want to travel any more and if they can't intercept you in London then you're sorry but you'll look forward with gritted teeth to seeing them another time.
It's really tricky that he has visa problems going to ireland - is there any way that he could get around it or is it convenient to not have it so that it forces you to go to see them (or so they think - not saying you should!).
I would jump at the chance of having 3 weeks with your mum, invite them for a few days to be polite, when they point out that they can't come, say what a shame you can't make it, will look forward to seeing you on a different trip/when you next come over to visit us - and then leave it at that. You've offered, they've refused. They've offered, you've refused. It's no ruder for you to refuse them than it is for them to refuse you. It's not your fault they can't get a visa to ireland. (Could they get one to Northern Ireland and could you and your mum maybe meet up with them for a couple of days there if it's not too tricky?)
And just keep repeating that it's not possible for you to visit paris this trip with such a young baby - you've given them options that work for you and the baby, if they can't do it then that's a shame but never mind.
Good luck - don't get brow beaten into going by your mil!