Congrat on the new baby!
Mirena seems to be compatable with nursing. I got to kellymom for most of my questions about nursing and here is what is on the site and the link to the info to read more if you feel like it
http://www.kellymom.com/health/meds/birthcontrol.html
Progestin-only contraceptives are the preferred choice for breastfeeding mothers when something hormonal is desired or necessary.
Progestin-only contraceptives come in several different forms:
the progestin-only pill (POP) also called the "mini-pill" (Micronor, Errin, Nor-QD, Ovrette, Microval, etc)
the birth control injection (Depo Provera)
the progesterone-releasing IUD (Mirena, Progestasert)
the birth control implant (Norplant, Implanon)
Milk supply: For most mothers, progestin-only forms of contraception do not cause problems with milk supply if started after the 6th-8th week postpartum and if given at normal doses. However, there are many reports (most anecdotal but nevertheless worth paying attention to) that some women do experience supply problems with these pills, so if you choose this method you still need to proceed with some caution.
If you're interested in one of the longer lasting progestin-only forms of birth control (the Depo-Provera shot lasts at least 12 weeks, but effects may be seen up to a year; the Mirena/Progestasert IUD and the Norplant implant can last up to 5 years), it may be a good idea to do a trial of progestin-only pills (mini-pill) for a month or more before deciding on the longer-term form of birth control. If you find that you are among the women whose supply drops significantly due to progestin-only birth control, you can simply discontinue the pills - rather than struggling with low milk supply for several months until the shot wears off or you get the implant or IUD removed.
Do note that the Mirena/Progestasert IUD delivers its hormone directly to the lining of the uterus, which only leads to a slight increase in progesterone levels in the blood stream (much lower than that found with the progesterone-only pill). As a result, there is much less chance of side effects from the progesterone than from the Depo-Provera shot or mini-pill.
Milk composition: At higher doses than normal this type of pill can affect the content of breastmilk. At these higher doses it has been shown to decrease the protein/nitrogen and lactose content of the milk. At regular doses, this does not seem to be as likely.
Safety: Progestin (progesterone) is approved by the AAP for use in breastfeeding mothers. See below for additional information on side effects related to lactation.