Somali "prepositions" (as they are called e.g. in Saeed 1993) always occur before the verb, perhaps along with various verbal object and subject pronouns, regardless of the position of a noun they may govern. Thus they might better be called "preverbal particles" than "prepositions."  There appear to be only eight such elements:
Relational or locative particles:
ú 'to, for'
'in, into, on, at, with, by means of'
'from, away from, out of'
'in company with'
Deictic particles:
sóo 'in the direction of the speaker'
síi 'away from the speaker'
wada 'together, towards each other (with plural subjects/object)'
kala 'apart, away from each other, separately'

Prepositional functions fulfilled by possessive phrases

Other preposition-like functions are fulfilled by verbs or nouns. One construction particularly worthy of note involves locative or temporal nouns with a possessive, used in effect as postpositions:

daarta gudeheeda 'inside the building', lit. 'the building her interior'
markabka gudihiisa 'inside the ship', lit. 'the ship his interior'
miiska dushiisa 'on top of the table', lit. 'the table his surface'
jid dhexdiisa 'in the middle of a road', lit. 'a road his center'
awrta dhexdooda , 'among the camels', lit.. 'the camels their center'
habeennimada horteeda 'before nightfall', lit. 'the night her front'

Among the nouns commonly used this way are:

gudo 'interior' gudihiisa gudeheeda 'inside, 
within (space or time)'
debed 'exerior' debeddiisa debeddeeda 'outside'
ag 'vicinity' agtiisa agteeda 'near'
dul '(top) surface' dushiisa dusheeda 'on top of'
hoos 'bottom' hoostiisa hoosteeda 'beneath'
gees 'side' geestiisa geesteeda 'beside'
hor 'front' hortiisa horteeda 'in front of, 
before (space or time)'
dabo 'back' dabadiisa dabadeeda 'behind, 
after (space or time)'
dar 'reason' dartiisa darteeda 'because of'
dhex 'center' dhexdiisa dhexdeeda 'in the middle of'
dhex 'center' dhexdooda dhexdooda 'among'

Note that the cited possessives end in the definite article, which may be missing or replaced by a different determiner:

muddo yar dabadeed 'after a little while' lit. 'period small his back'

It appears that the head of these constructions is the second element (i.e. the possessed locative or temporal noun), at least based on the analogy with similar constructions:

Cali walaashiis 'Ali's sister', lit. 'Ali his sister'

Combinations of particles and pronouns

Locative/relational particles combine with each other and with various object and subject pronouns, sometimes with phonological changes, with results that are written as single words.

Two-particle combinations:

ú ugú ugú ugá ulá
kagá kagá kulá
kagá kalá

Verbal object pronoun + particle combinations:

The "prepositions" also combine phonologically with object pronouns, which precede them, and with the impersonal subject pronoun /la/.
i ('me') íi igú igá ilá
ku ('you') kúu kugú káa kulá
na ('we [excl.]') nóo nagú nagá nalá
ina ('we [incl.]') inóo inagú inagá inalá
idin ('you [pl.]') idíin idinkú idinká idinlá
la ('one [subj.]') lóo lagú lagá lalá

Combinations of pronouns and particle clusters

ugú ugá ulá kagá kulá kalá
i iigú iigá iilá igagá igulá igalá
ku kuugú kaagá kuulá kaagá kugulá kaalá
na noogú noogá noolá nagagá nagulá nagalá
ina inoogú inoogá inoolá inagagá inagulá inagalá
idin idiinkú idiinká idiinlá idinkagá idinkulá idinkalá
la loogú loogá loolá lagagá lagulá lagalá