Can a trustee and beneficiary of a irrevocable trust gift money to the same trust, if so how much is allowed and does the trust have to pay taxes on such a gift ?

[M-MTax]

The grantor can add money to the trust unless there is some specific prohibition which is not very likely.

It's not a gift if you give something to yourself. This would be nothing more than a deposit......a transfer of money to the trust which the trustee-bene already owns.