"The woman is on the television."
Translation:Tá an bhean ar an teilifís.
When preceded by prepositions ag, ar, faoi, le, ó, roimh, thar, trí, um eclipses (b, c, f, g, p) or lenites (b, c, f, g, m, p)
"t" isn't in (b, c, f, g, p), therefore teilifís isn't eclipsed after ar an. Neither are words that begin with d.
Note that it would be ar an dteilifís in Munster Irish, but in Ulster Irish, where they lenite instead of eclipsing, teilifís wouldn't be lenited because of DeNTaLS-DoTS.