"Scríobhaim an leabhar."
Translation:I write the book.
What fascinates me most about Irish is that you can trace impossibly well when a certain word becam important in Irish history. Some animals were not 'known' before the opening towards the world, so their 'stem-language' is English. (f.e eilifint (elephant) in contrast to madra (dog) ) Writing, however, corresponds to the famous Irish Center of Christian book production (Book of Kells/Armagh etc.), so the stem-language for the word 'write' is latin 'scrivere'.
eDIL notes that elef(a)int (the Middle Irish form of eilifint ) came from an Irish translation of Lucan’s Pharsalia, so English wasn’t involved; it came to Middle Irish directly from Latin elephantus. (Its form in the Pharsalia was elephans, a third declension variant of elephantus that was closer to its Greek source ἐλέφᾱς.)
Hipopotamus comes from the Greek hippopótamos "riverine horse".
I wonder when that came into the Irish language :-)
(There is a famous, or infamous, statue of a rhinoceros in the river Dodder in south Dublin. Nobody knows if it was supposed to be a hippopotamus).
Is there a reason "write" and "am writing" are not interchangeable here?
Yes. Irish, like English, has a separate present progressive and present habitual form.