The Irumudi is the bag consisting of two compartments carried by the Ayyappa devotees on their head while visiting the Sabarimala Ayyappa temple. The first compartment contains sacred coconut which is filled with ghee and other articles required for the worship and the other compartment is reserved for personal belongings.
The coconut is pierced in one of the holes and ghee is filled into it. Then it is closed with a cork and packed into the irumudi along with some rice, jaggery and coins as offerings to the Lord. This has a lot of significance in which the ghee represents the atman and the coconut represents the body.
The coconut having three eyes represents the intellect and spiritual eyes. When the devotees break open the coconut at Sabarimala Ayyappa Temple and pour the ghee over the Lord, it symbolises the merging of the soul with the lord and discarding the body. Then one part of the coconut is put into the homa kunda to signify this fact.
The other part of the compartment in Irumudi is for personal belongings and this represents the prarabhdha karma or the accumulation of the merits and demerits obtained from previous births. When you use up those items, you are burning the karma and this helps you get closer to moksha.
This is the significance of Irumudi while visiting the Sabarimala Ayyappa Temple. It is for this reason that only devotees carrying Irumudi on their head are allowed to climb the eighteen sacred steps at the temple.