Tinkerbell, I would switch to Oestrogel immediately! Having used the Mirena coil with patches myself and failing miserably, I would not personally recommend this combination. While I was on that particular regime, my oestrogen blood levels were pathetic even when I upped my dose to the 100mcg patch! Having used both oestrogen patches and Oestrogel, I think the gel is a far more effective, user friendly product with far better absorption rates.
I'm afraid I had constant vaginal irritation on this regime but again, that was probably due to lack of oestrogen.
I think the Mirena/Oestrogel combination would work very well because you have the flexibility to increase or decrease the oestrogen dose very quickly and easily. You will probably need 3 pumps of gel everyday to get good results because although most of the progesterone from the coil is confined to the womb, a fair amount of progesterone will get into your overall system meaning you need more oestrogen to get symptom relief.
I have carried out an experiment over the last month or so - I put one pump of gel on my upper, inner thighs everyday and it has really helped with (minor) irritation down below so that is another added bonus. I can't guarantee it will work for everyone but it's worth a try.