RWY INCURSION AT THE END OF A LONG DAY.

Narrative

WHILE TAXIING FOR TKOF RWY 19 ON TWR FREQ; WAS ISSUED A SID CHANGE; WHILE CONDUCTING TAXI CHKLIST. I GOT BACKED UP AND HAD MY HEAD DOWN WHEN PF; TAXIING; STATED 'WE WERE CLRED TO HOLD SHORT OF RWY 19; SO WE'RE CLRED TO CROSS RWY 24.' I LOOKED UP MOMENTARILY AND SAW RWY THRESHOLD; AND SAID 'YEAH; WE'RE CLRED TO RWY 19.' WE TAXIED ACROSS RWY THRESHOLD; VISUALLY CLRING L AND R; WHEN TWR STATED THAT WE HAD JUST CROSSED RWY 19; AND 'WHAT WERE WE DOING?' WE MADE A 180 DEG TURN ON TXWY AND HELD SHORT OF RWY 19. THERE WAS TFC ON ABOUT A 5 MI FINAL (?) TO RWY 19 AT THE TIME. WE HAD STARTED THE NIGHT AT HEATHROW ARPT (EGLL) AND REPOSITIONED TO PARIS (LRPB) FOR A PAX FLT; AND HAD JUST DROPPED PAX AT TEB. WE WERE TAXIING FOR TKOF TO OUR HOME BASE (ISP). AT THE TIME OF THE INCIDENT; WE HAD BEEN ON DUTY 12 1/2 HRS; AND HAD FLOWN 8 HRS 25 MINS. THIS IS NOT AN ESPECIALLY LONG DAY FOR OUR OP; BUT AFTER THIS INCIDENT; THE COPLT (ALSO CAPT RATED) AND I ADMITTED WE WERE VERY TIRED. WE HAD DEPARTED TEB FOR EGLL ON JAN/TUE/02 AND HAD ARRIVED AT ABOUT XA00Z ON JAN/WED/02. THERE WERE CONSTRUCTION BARRIERS ON TXWY JUST BEFORE RWY 19 THRESHOLD; WHICH MAY HAVE DISTR PLT TAXIING.