>>149984596
The high ends for Gamera put him at around 1200 tons. Even if we had Gamera and Irys scale to Godzilla's mass, Destoroyah should still be much stronger and has all sorts of abilities that would kill Irys. IIRC, Irys never had much of a healing factor, while Destoroyah was overpowering Burning Godzilla despite the latter healing wounds almost instantly.
Destoroyah is also far tankier than Irys, considering how he was able to endure multiple blasts from Godzilla's spiral beam (which annihilated Space Godzilla). Extreme temperatures are even stated to be Destoroyah's weakness.