When it comes to somebody it is quite clearly to be translated with jemand, irgendjemand or irgendwer.

The declination of jemand corresponds to the declination of niemand (see chapter 12.8). Jemand is - just like niemand - only to be used in singular, is independent of gender and it can only be used as a noun.

Nominative: Genitive: Dative: Accusative : jemand jemandes * jemandem jemanden

Irgendjemand is declined identically to jemand.

Nominative: Genitive: Dative: Accusative : irgendjemand irgendjemandes * irgendjemandem irgendjemanden

Then there is irgendwer to be declined. Here as well it is can only be used as a noun, is independent of gender and is only to be used in singular.

Nominative: Genitive: Dative: Accusative : irgendwer irgendwessen* irgendwem irgendwen

* All three Genitive-forms are grammatically correct, but hardly ever used. Therefore we present them, but you can forget them the next second.

They are synonyms that can be used in each others places.

Someone / somebody has to tell him. (Nominative)

Jemand muss es ihm sagen. Irgendjemand muss es ihm sagen. Irgendwer muss es ihm sagen.

If using irgendwer or irgendjemand then the emphasis more on the fact that it doesn't matter who it is as long as there is someone.

I give money to someone / somebody (Dative)

Ich gebe jemandem Geld. Ich gebe irgendjemandem Geld. Ich gebe irgendwem Geld.

I see someone / somebody (Accusative) Ich sehe jemanden. Ich sehe irgendjemanden. Ich sehe irgendwen.

For jemand and irgendjemand the same proves right as for niemand. The Dative and the Accusative are being used less and less. The Duden accepts also the use of the basic form in direct and indirect object form.

examples

Dative: Accusative : Ich gebe es jemandem. Ich gebe es jemand. Ich gebe es irgendjemandem. Ich gebe es irgendjemand. Ich sehe jemanden. Ich sehe jemand. Ich sehe irgendjemanden. Ich sehe irgendjemand.