Had to look up what mirabelles were.
I think we have a tree with fruit like this in our neglected little orchard. The tree is really quite big and healthy looking but only bears a handful of fruit each year and often so high that it's not worth struggling for. Asked neighbours but no-one knew the name of it .... though thought it was edible. Did try them and I'm still alive.
How would you encourage the tree to fruit more? Probably asking in the wrong place but will give it a go
It does sound like a Mirabelle. If it has a plum type stone then it will be. I don't think it's a cross HGL, just called cherry plum because when it's ripe half the trees have bright red fruit (the others have yellow fruit.) I think it's a true plum which originates from somewhere towards Russia....'over there somewhere'
Ours seem to be sporadic fruiters, and I think that's why they're called Mirabelles. You don't tend to notice the fruit before they're ripe, then suddenly one day they are absolutely covered in beautiful glowing red, ripe fruit, which taste like plums.. They flower a bit early so if there's a frost or chilling wind then they won't set much fruit, although in general they are amazingly hardy.
We haven't tried pruning or pollarding ours, but the fruit being so high is awkward. I think that maybe if you follow the timing rules for pruning plums it should be ok. They are not on a rootstock so any regrowth will still be Mirabelle.
For jam, removing the stones takes forever, so any for home consumption keeps the stones in. It makes a lovely red jam - even the yellow ones.