"He left my child at the park."
Translation:D'fhág sé mo pháiste ag an bpáirc.
Try this, it could help.
Write a short story, where you will aim at using the main prepositions (ar, ag, i, ó, faoi, le) in all the combinations you can manage to fit it. Make sure the nouns you are using in the story cover all the initial letters that can be impacted, and some key exceptions or alternatives. Learn it by heart, record yourself and then use the recording to dictate it back to yourself, re-write it from memory. This is the principle of the French "dictée" that school children hate but parents love! It is an auto-dictée.
It might involve girls jumping over fences in the park, boys hiding under a table in the house, cats being lost in a park and found again under the table, etc.
Try to make it as short and condensed as possible.
Correcting yourself is the best way, as you are always at hand to do it.
Alternative: pick one article from http://www.rte.ie/news/nuacht/ a day, print it, and highlight the effect of prepositions. Try to spot "mistakes"! Share if you are in doubt.