The Monster 3D from Diamond was quite a famous card. This is due to the fact that it's probably the most sold add-on 3D card with 3Dfx Voodoo chip on the market. It shares its features with most of the other 4 MB Voodoo cards, the difference is mainly determined by the games it is sold with and the connector cable, which Diamond lately changed to a very high quality one.
To say it again, this card is used as addition to your existing graphic card, using up another PCI slot, but don't worry about IRQs, the Monster 3D doesn't need one. The card needs to be connected in between your exisiting graphic card and the monitor, using a special cable to connect the output of your normal graphic adapter with the 2D input of the Monster 3D. This procedure is the same as with any other Voodoo card, the only difference is the quality of the connection cable. If it is of low quality, it can degrade the quality of the 2D signal of your 2D graphic adapter. Diamond is lately using a very thick high quality cable.