Cén fáth a deirimid ''roimhe seo'' in ionad ''roimh seo''? Tá mearbhall orm.
roimh é seo = roimhe seo.
Same as why Ag déanamh é seo should be á dhéanamh seo
The roimhe in roimhe seo is an adverb, as opposed to the prepositional pronoun roimhe. Roimh is a preposition rather than an adverb. The preposition can be used in an adverbial phrase either temporally or locationally; the adverb seems only to be used temporally.
eDIL shows that the preposition is the original word, and the adverb evolved from one of the Middle Irish forms of what became the prepositional pronoun roimhe. Before the adverb took its current roimhe form, its immediately prior spelling was roime, and this spelling can be found as a variant in Dinneen’s dictionary. The adverb and the prepositional pronoun took alternative spelling evolutions from Middle Irish before winding up with identical spellings in modern Irish.