What is the ruling on offering the regular Sunnah prayer that is usually offered after the obligatory prayer, before the obligatory prayer?

The regular Sunnah prayers are of two types: 

1.The Sunnah prayers that are
offered before the obligatory prayer. They are: two rak‘ahs before Fajr and
four rak‘ahs with two tasleems (i.e., offered two by two) before Zuhr.

The time for this type of Sunnah prayer begins when the time
for that prayer begins, and lasts until one begins to offer the obligatory
prayer. 

2.The Sunnah prayers that are
offered after the obligatory prayer. They are: two rak‘ahs after Maghrib,
two rak‘ahs after ‘Isha’ and two rak‘ahs after Zuhr.

The time for this type of Sunnah prayer begins when the
obligatory prayer is over and lasts until the end of the time for that
prayer. 

Ibn Qudaamah (may Allah have mercy on him) said: With regard
to every Sunnah prayer that is offered before the obligatory prayer, its
time lasts from the beginning of the time for that prayer until the start of
the obligatory prayer. With regard to every Sunnah prayer that is offered
after the obligatory prayer, its time lasts from when that prayer is
finished until the end of that time.

End quote from al-Mughni, 1/436 

It says in al-Mawsoo‘ah al-Fiqhiyyah (25/281-282): The
regular Sunnah prayers are connected to the obligatory prayers; some of them
are offered before the obligatory prayer, such as the Sunnah prayer of Fajr
and the Sunnah prayer that comes before Zuhr. And some of them are offered
after the obligatory prayer, such as the Sunnah prayer that comes after
Zuhr, the Sunnah prayers of Maghrib and ‘Isha’, Witr, and qiyaam in Ramadan
(i.e., Taraweeh).

With regard to whatever of these Sunnah prayers come before
the obligatory prayer, the time for them begins when the time for the
obligatory prayer begins, and ends when the iqaamah for the obligatory
prayer is given, if that iqaamah is given in a group or congregation
(jamaa‘ah), because once the iqaamah is given for prayer, there is no prayer
except the prescribed obligatory prayer. But if an individual is going to
offer the prayer on his own, the time for the Sunnah prayer lasts until he
begins the obligatory prayer. 

With regard to the Sunnah prayers that come after the
obligatory prayer, such as the Sunnah prayer that comes after Zuhr and the
Sunnah prayers of Maghrib and ‘Isha’, the time for each of them begins when
the obligatory prayer has been completed and lasts until the time of the
obligatory prayer ends and the time for the next prayer begins.

End quote. 

Based on the above, if a person offers the Sunnah prayer that
should be offered after ‘Isha’ before he prays ‘Isha’, it is as if he has
performed that Sunnah before its time. Hence it will not be counted as a
regular sunnah prayer; rather it is a naafil prayer between the adhaan and
iqaamah, for which one earns the reward of a naafil prayer, not a regular
Sunnah prayer. 

An-Nawawi (may Allah have mercy on him) said: It is mustahabb
to pray two rak‘ahs or more before ‘Isha’, because of the hadeeth of
‘Abdullah ibn Maghfal, according to which the Prophet (blessings and peace
of Allah be upon him) said: “Between each two calls (the adhaan and iqaamah)
there should be a prayer, between each two calls there should be a prayer,
between each two calls there should be a prayer” – and the third time he
said, “for whoever wishes.” Narrated by al-Bukhaari and Muslim. What is
meant by the two calls is the adhaan and iqaamah, according to scholarly
consensus.

End quote from al-Majmoo‘, 3/504 

For more information please see the answer to question no.
128164

With regard to the one who used to do that and was unaware of
the ruling as mentioned above, then we hope that Allah, by His bounty and
grace, will grant him the reward of one who offered the regular Sunnah
prayer, because he was unaware of the ruling on that matter.

And Allah knows best.