The expression “explanatory relative sentence” means here such relative sentences as “the man who spoke to you“, i.e. the man who is defined by the fact that he spoke to you, what we know about him (to start with) is the fact that he spoke to you.
Let’s say, for instance, “The man who spoke to you wrote this novel”. In Irish, this translates into Scríobh an fear a labhair leat an t-úrscéal seo. That’s fine, but the explanatory relative clause is here inside the main clause: Scríobh an fear [a labhair leat] an t-úrscéal seo. In this example it is easy enough to tell the relative clause from the main clause, but in long and complicated sentences it might become more difficult.
Instead, it is advisable to write:
An fear a labhair leat, scríobh sé an t-úrscéal seo “The man who spoke to you, he wrote this novel”
An bhean atá ag canadh amhráin, is léise an gluaisteán úd thall “The automobile over there belongs to the lady who is singing” (“The woman who is singing, the automobile over there belongs to her”)
An chuid ba mhó de na scríbhneoirí Sóivéadacha a chuaigh ag scríobh úrscéalta i ndiaidh an chogaidh, bhí siad ina n-iriseoirí cathéadain lena linn “Most of the Soviet writers who went to write war novels after the war were front journalists during it.”
An fear a chéadcheap Teoiric na Coibhneasachta, is é sin Albert Einstein, chaith sé tréimhse ag obair in Oifig na bPaitinní san Eilvéis. “The man who invented the theory of relativity, that is, Albert Einstein, – he spent some time working in the Swiss Patent Office.”