I think the song goes a bit like this:

Sé mo laoch, mo Ghile Mear
Sé mo Chaesar Gile Mear
Suan ná séan ní bhfuaireas féin
Ó chuaigh i gcéin mo Ghile Mear


Grief and pain are all I know
my heart is sore my tears aflow
we saw him go our buachaill beo
no word we know of him och on

Sé mo laoch, mo Ghile Mear
Sé mo Chaesar Gile Mear
Suan ná séan ní bhfuaireas féin
Ó chuaigh i gcéin mo Ghile Mear

A proud and gallant chevalier
a high born scion of gentle mien
a fiery blade engaged to lead
he'd break the bravest in the field

Sé mo laoch, mo Ghile Mear
Sé mo Chaesar Gile Mear
Suan ná séan ní bhfuaireas féin
Ó chuaigh i gcéin mo Ghile Mear

Then we'll sing his praise as sweet harps play
and proudly toast his noble fame
with spirit and with mind aflame
so wish him strength and length of day

Sé mo laoch, mo Ghile Mear
Sé mo Chaesar Gile Mear
Suan ná séan ní bhfuaireas féin
Ó chuaigh i gcéin mo Ghile Mear (Repeated)